Why does Earth have seasons? It’s natural to think our world’s seasons result from Earth’s changing distance from the sun. But you can easily understand that’s not the case, when you realize that Earth is farther from the sun in July (northern summer) and closer in January (northern winter). The fact that Earth’s Northern and Southern Hemispheres have their summers and winters at opposite times of the year provides a clue to the real reason for seasons: that reason is Earth’s 23 1/2 degree tilt on its axis. The photos and video on this page – from NASA – show Earth’s solstices and equinoxes from space.
They can help you visualize why our seasons unfold as they do, continuously, throughout each year.
EUMETSAT‘s Meteosat-9 (a weather satellite) captured the four views below of Earth from geosynchronous orbit in 2010 and 2011. A satellite in geosynchronous orbit stays over the same point on Earth all the time. The images show how sunlight fell on the Earth on December 21, 2010 (upper left), March 20, 2011 (upper right), June 21, 2011 (lower left), and September 20, 2011 (lower right). Each image was taken at 6:12 a.m. local time.
Around 6 a.m. local time each day, the sun, Earth, and any geosynchronous satellite form a right angle, affording straight-down view of Earth’s terminator line, that is, the line between our world’s day and night sides. The shape of this line between night and day varies with the seasons, which means different lengths of days and differing amounts of warming sunshine.
The line is actually a curve because the Earth is round, but satellite images show it in two dimensions only.
On March 20 and September 20, the terminator is a straight north-south line, and the sun is said to sit directly above the equator. On December 21, the sun resides directly over the Tropic of Capricorn when viewed from the ground, and sunlight spreads over more of the Southern Hemisphere. On June 21, the sun sits above the Tropic of Cancer, spreading more sunlight in the north.
Earth’s seasons result from our planet’s tilt on its axis with respect to our orbit around the sun. Upper left: northern winter solstice. Lower left: northern summer solstice. Upper right: northern spring equinox. Lower right: northern autumnal equinox. Images from EUMETSAT’s Meteosat-9 weather satellite, via the archives of NASA Earth Observatory.
What’s causing all this change? It’s tempting to imagine it’s the sun moving north or south through the seasons. But that’s not it. Instead, the change in the orientation and angles between the Earth and the sun result from Earth’s never-ending motion in orbit around the sun.
The axis of the Earth is tilted 23 1/2 degrees relative to the sun and the ecliptic plane. The axis is tilted away from the sun at the December solstice and toward the sun at the June solstice, spreading more and less light on each hemisphere. At the equinoxes, the tilt is at a right angle to the sun and the light is spread evenly.
Image via NASA.
Bottom line: A video from NASA shows how sunlight falls on Earth’s surface during the solstices and equinoxes, as seen by the weather satellite Meteosat-9 in 2010 and 2011.
In this illustration of a pterodactyl nesting ground 124 million years ago, a hatchling (flapling) pterodactyl emerges from the sand and gazes at the sky for the first time. Other hatchlings lie exhausted from their struggles or crawl to safety on trees. The less lucky are caught and eaten. From the safety of the trees, flaplings make their maiden flights. Image via James Brown.
New research has found that pterodactyls, extinct flying reptiles that lived during the time of the dinosaurs, could fly from birth. It’s an ability that no other flying animal living today, or in the history of life as we know it, has been able to replicate.
Previously, pterodactyls were thought to only be able to fly once they had grown to almost full size, like birds or bats, which have to learn how to use their wings.
Theoretically what pterosaurs did, growing and flying, is impossible, but they didn’t know this, so they did it anyway.
Another fundamental difference between baby pterodactyls – known as flaplings – and baby birds or bats, is that they had no parental care and had to feed and look after themselves from birth. The researchers suggest that the flaplings’ ability to fly gave them a lifesaving survival mechanism which they used to evade carnivorous dinosaurs. On the other hand, the researchers said, this same ability also proved to be one of their biggest killers, as the demanding and dangerous process of flight led to many of them dying at a very early age.
Previous studies were based on fossilized embryos of the creatures that had poorly developed wings. For the new study, the researchers compared these embryos with data on prenatal growth in birds and crocodiles, which suggested that they were still at an early stage of development and a long way from hatching. The discovery of more advanced pterodactyl embryos in China and Argentina that died just before they hatched provided the evidence that pterodactyls had the ability to fly from birth.
Artist’s illustration of a hadrosaur, and its hypothesized nose. Look below for an actual photo of hadrosaur bones. Image via Julius Csotonyi/University of Calgary.
Researchers studied fossils of juveniles to help work out how the head of an adult Prosaurolophus maximus dinosaur might have looked.
Prosaurolophus was a hadrosaur that lived 75 million years ago in what’s now northern Montana and southern Alberta, Canada. Unlike many duck-bills, which had a large bony crest on their heads, Prosaurolophus had only a small crest on the forehead. The researchers were interested in determining how the crest changed as the animal grew, as this feature has been thought to be related to sexual maturity and mate attraction.
The fossils used in the study – from Alberta’s Royal Tyrrell Museum – are the youngest and smallest individuals known for the species.
Front half of the skeleton of a juvenile Prosaurolophus dinosaur. Image via Eamon Drysdale, specimen at the Royal Tyrrell Museum.
The study suggests that a showy snout and bony forehead crest developed as the animal matured. Eamon Drysdale is a graduate student in the Department of Geoscience at University of Calgary, and the lead author of the study published May 19, 2019, in the Journal of Vertebrate Paleontology. Drysdale said in a statement:
We noticed that the bony crest grew very slowly in Prosaurolophus and remained small, unlike what happened in some duck-bills, which rapidly developed a large bony crest. Instead, rapid changes in the snout as the animal matured suggest that a soft tissue structure may have associated with the nostrils and used for display.
In Prosaurolophus, the snout would have been the primary display feature rather than the large head crest of other duck-bills.
The research team with the fossil skeleton of a duck-billed dinosaur Prosaurolophus in collections at Royal Tyrrell Museum, that was found in ancient oceanic sediments. Image via Royal Tyrrell Museum of Palaeontology.
This idea of a showy, fleshy snout had been hypothesized previously by paleontologists, and the new study now offers evidence to support this premise. Darla Zelenitsky is assistant professor in the University of Calgary Department of Geoscience, and Drysdale’s supervisor. She said:
Gathering evidence to support this hypothesis was difficult because it required finding the fossils of juvenile individuals, which are very rare. It took over 30 years for the Royal Tyrrell Museum to recover a decent growth series for the species.
The unusual setting in which the fossils were discovered also shed light on the environments in which Prosaurolophus lived.
Dinosaurs were land dwellers, so their remains are usually found in rocks deposited in rivers and lakes. This was not the case for the three juvenile Prosaurolophus fossils featured in this study, said the researchers, which were found in muds deposited at the bottom of an inland sea that covered Alberta about 75 million years ago. François Therrien, curator at the Royal Tyrrell Museum, also supervised Drysdale. He said:
It was very fortuitous that three young individuals of the same dinosaur species happened to float out to sea and sink to the bottom where they were buried. Their preservation in the fine mud contributed to the fossilization of large patches of skin showing the flanks of these animals were covered in a mosaic of large and small scales.
As for why and how these dinosaurs ended up at sea, Drysdale said:
It may have been that these particular dinosaurs spent a lot of time in coastal areas. Living close to the coast, these animals may have been more easily washed out to sea after they died.
Bottom line: A study used fossils from juvenile dinosaurs to help determine the appearance of the hadrosaur.
Above photo: June solstice sunset in the nation of Oman, on the Arabian Peninsula, from our friend Priya Kumar. Thank you, Priya!
Here’s a natural phenomenon you might never have imagined. That is, the sun actually takes more time to set around the time of a solstice.
It’s true. The longest sunsets (and sunrises) occur at or near the solstices. The shortest sunsets (and sunrises) occur at or near the equinoxes. This is true whether you live in the Northern or Southern Hemisphere.
And, by the way, when we say sunset here, we’re talking about the actual number of minutes it takes for the body of the sun to sink below the western horizon.
Adrian Strand captured this photo on a beach in northwest England.
When is the solstice? In 2019, the Northern Hemisphere’s summer solstice – and Southern Hemisphere’s winter solstice – will fall on June 21 at 15:54 UTC.
In the United States, that translates to June 21 at 11:54 a.m EDT, 10:54 a.m. CDT, 9:54 a.m. MDT, 8:54 a.m. PDT, 7:54 a.m. Alaska Daylight Time and 5:54 a.m. Hawaii-Aleutian Daylight Time. Translate to your time zone.
Equinoxes and solstices, via Geosync. The Earth’s axis points straight up and down, with north at the top. The solstices are on the left (December solstice at top, June solstice at bottom) and the equinoxes are to the right (March equinox at top. September equinox at bottom).
Why is the sunset longer around the solstice? As viewed from both the Northern and Southern Hemispheres, the sun rises and sets farthest north at the June solstice and farthest south at the December solstice.
Now consider that the farther the sun sets from due west along the horizon, the shallower the angle of the setting sun. That means a longer duration for sunset at the solstices.
Meanwhile, at an equinox, the sun rises due east and sets due west. That means – on the day of an equinox – the setting sun hits the horizon at its steepest possible angle.
The sunset duration varies by latitude, but let’s just consider one latitude, 40 degrees north, the latitude of Denver or Philadelphia in the United States, or Beijing in China. At that latitude, on the day of a solstice, the sun sets in about 3 1/4 minutes.
On the other hand, at 40 degrees north latitude, the equinox sun sets in roughly 2 3/4 minutes.
At more northerly temperate latitudes, the sunset duration is greater; and at latitudes closer to the equator, the sunset duration is less. Near the Arctic Circle (65 degrees north latitude), the duration of a solstice sunset lasts about 15 minutes. At the equator (0 degrees latitude), the solstice sun takes a little over 2 1/4 minutes to set.
Regardless of latitude, however, the duration of sunset is always longest at or near the solstices.
As it turns out, the sunset and sunrise are a tad longer on a December solstice than they are on a June solstice. That’s because the sun is closer to Earth in December than it is in June. Therefore, the sun’s disk looms a bit larger in our sky in December, and so it takes slightly longer to set.
Additionally, the closer December sun moves eastward upon the ecliptic at a faster clip, helping to slow down the December solstice sunset (and sunrise) even more. For instance, at 50 degrees north latitude, the winter solstice sunset (sunrise) lasts about 4 minutes and 18 seconds, or about 8 seconds longer than the sunset (sunrise) on the summer solstice.
Bottom line: Here’s a natural phenomenon you might never have imagined. That is, the longest sunsets happen around the time of a solstice.
John Ashley – who has contributed many wonderful sky photos to EarthSky – took this underwater photo May 31, 2019. He wrote: “Two female Westslope cutthroat trout argue over a favorite location.”
Here’s another May 31 photo by John Ashley. He wrote: “A female Westslope cutthroat trout tail-slaps another in a disagreement over a favorite area.” These fish, by the way, are the official state fish for Montana.
John Ashley wrote of this May 31, 2019, photo: “A Westslope cutthroat trout floats motionless in a calm pool on a Montana spawning stream.”
Bottom line: Photos of spawning Westslope cutthroat trout. The U.S. Fish and Wildlife Service (FWS) was petitioned to list the Westslope cutthroat trout under the protection of the Endangered Species Act. In 2000, the FWS determined that listing the Westslope cutthroat trout was not warranted because of its wide distribution and the available habitat on public lands. Conservation efforts by state and federal agencies are currently underway to restore Westslope cutthroat trout. Read more.
The South Pole-Aitken Basin (outlined) on the far side of the moon. The unusual mass is beneath the surface in this area. Image via NASA.
What is hiding beneath the largest crater on Earth’s moon (in fact, the largest crater in our solar system)? That’s what scientists said they’d like to find out after an unusual large mass of material was discovered lurking underneath the lunar South Pole-Aitken Basin. It’s a lot of mass, too, according to Peter B. James, assistant professor of planetary geophysics in Baylor University’s College of Arts & Sciences:
Imagine taking a pile of metal five times larger than the Big Island of Hawaii and burying it underground. That’s roughly how much unexpected mass we detected.
The intriguing peer-reviewed findings were first published in the April 15, 2019, issue of the journal Geophysical Research Letters. From the abstract:
The South Pole-Aitken Basin is a gigantic impact structure on the far side of the moon, with an inner rim extending approximately 2,000 kilometers [1,200 miles] in the long-axis dimension. The structure and history of this basin are illuminated by gravity and topography data, which constrain the subsurface distribution of mass. These data point to the existence of a large excess of mass in the moon’s mantle under the South Pole-Aitken Basin. This anomaly … likely extends to depths of more than 300 km [about 200 miles].
False-color map of the far side of the moon, showing the location of the unusual massive subsurface deposit beneath the South Pole-Aitken Basin. Image via NASA Goddard Space Flight Center/ University of Arizona/ Baylor University.
So what is this mysterious mass?
It is most likely metal of some kind, given its density and the fact that it is weighing the crater basin floor down by more than half a mile (0.8 km). An ancient asteroid impact would be a logical solution. Computer simulations of large asteroid impacts suggest that, under the right conditions, an iron-nickel core of an asteroid might be lodged into the upper mantle of the moon (the layer between the moon’s crust and core) during an impact, in this case the impact that created the South Pole-Aitken Basin.
Researchers analyzed data from spacecraft used for NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission to measure very small changes in gravity around the moon. As James explained:
When we combined that with lunar topography data from the Lunar Reconnaissance Orbiter (LRO), we discovered the unexpectedly large amount of mass hundreds of miles underneath the South Pole-Aitken basin. One of the explanations of this extra mass is that the metal from the asteroid that formed this crater is still embedded in the moon’s mantle. We did the math and showed that a sufficiently dispersed core of the asteroid that made the impact could remain suspended in the moon’s mantle until the present day, rather than sinking to the moon’s core.
The South Pole-Aitken Basin is estimated to have been formed about 4 billion years ago. The solar system was a very chaotic place back then, with collisions occurring between rocky and metallic bodies such as asteroids and young protoplanets – planetary embryos – on a pretty much regular basis. It seems quite feasible, then, that this is how the dense subsurface mass on the moon got there.
One other plausible theory, however, is that the mass might be a concentration of dense oxides associated with the last stage of lunar magma ocean solidification. It is theorized that the moon once had an ocean of sorts – not of water, but of magma, or molten rock – which then cooled and solidified. In the process, the oxides could have been deposited in this region, forming the large mass.
These scientists say an asteroid impact is still the leading hypothesis, however, and James referred to the South Pole-Aitken Basin as one of the best natural laboratories for studying catastrophic impacts in the early solar system.
The South Pole-Aitken Basis is the largest known crater in the solar system. Measured from outer rim to outer rim, it’s about 1,600 miles (2,500 km) in diameter and 8.1 miles (13 km) deep. It was named for two features on opposite sides of the basin: Aitken Crater on the northern end and the lunar south pole at the other end. The basin’s existence had been suspected since 1962, based on data from the Luna 3 and Zond 3 orbiters, but was not confirmed until the mid-1960s by the Lunar Orbiter program.
On January 3, 2019, China’s Chang’e 4 spacecraft landed within this basin, in the smaller and younger Von Kármán Crater. This was the first time that any spacecraft has landed on the far side of the moon. It has studied samples of material thought to have come from deeper within the moon’s mantle, excavated during the impact that created the crater. This is a unique opportunity to explore in detail not only the crater, but a small portion of the larger basin as well.
In recent years, astronomers probing the edges of the Milky Way have observed very strong explosions on stars, which they’ve dubbed “superflares,” that have energies up to 10,000 times that of typical solar flares.
Superflares happen when stars – for reasons that scientists still don’t understand- eject huge bursts of energy that can be seen from hundreds of light years away. Until recently, researchers assumed that such explosions occurred mostly on stars that, unlike Earth’s sun, were young and active.
Now, new research suggests superflares can also occur on older, quieter stars like our own sun, albeit more rarely, perhaps once every few thousand years.
University of Colorado researcher Yuta Notsu is the lead author of the peer-reviewed study, published May 3, 2019, in The Astrophysical Journal. Notsu said the study results should be a wake-up call for life on our planet. That’s because if a superflare erupted from the sun, he said, Earth would likely sit in the path of a wave of high-energy radiation. Such a blast could disrupt electronics across the globe, causing widespread blackouts and shorting out communication satellites in orbit. Notsu said in a statement:
Our study shows that superflares are rare events. But there is some possibility that we could experience such an event in the next 100 years or so.
Scientists first discovered superflares via NASA’s Kepler Space Telescope. The spacecraft, which looks for exoplanets circling distant stars, also found something odd about those stars themselves. In rare events, the light from distant stars seemed to get suddenly, and momentarily, brighter.
Notsu explained that normal-sized flares are common on the sun. But what the Kepler data was showing seemed to be much bigger, on the order of hundreds to thousands of times more powerful than the largest flare ever recorded with modern instruments on Earth. And, Notsu said, that data raised an obvious question: Could a superflare also occur on our own sun? Notsu said:
When our sun was young, it was very active because it rotated very fast and probably generated more powerful flares. But we didn’t know if such large flares occur on the modern sun with very low frequency.
To find out, Notsu and an international team of researchers turned to data on superflares from the European Space Agency’s Gaia spacecraft and from the Apache Point Observatory in New Mexico. Based on the team’s calculations, younger stars tend to produce the most superflares. But older stars like our sun, at 4.6 billion years old, aren’t off the hook. Notsu said:
Young stars have superflares once every week or so. For the sun, it’s once every few thousand years on average.
Notsu can’t be sure when the next big solar light show is due to hit Earth. But he said that it’s a matter of when, not if. Still, that could give humans time to prepare, protecting electronics on the ground and in orbit from radiation in space. He said:
If a superflare occurred 1,000 years ago, it was probably no big problem. People may have seen a large aurora. Now, it’s a much bigger problem because of our electronics.
Bottom line: New research suggests a superflare could happen on our sun.
Circumpolar stars stay above the horizon all hours of the day, every day of the year. These stars neither rise nor set but always remain in our sky. Even when you can’t see them – when the sun is out and it’s daytime – these stars are up there, circling endlessly around the sky’s north or south pole.
For instance, the stars of the famous Big Dipper asterism are circumpolar at all latitudes north of 41 degrees north latitude, which includes the northern half of the mainland United States and most of Europe.
From the northern U.S., Canada or similar latitudes, the Big Dipper is circumpolar, always above your horizon. Image shows Big Dipper at midnight at various seasons. “Spring up and fall down” for the Dipper’s appearance in our northern sky. It ascends in the northeast on spring evenings and descends in the northwest on fall evenings. Image via Night Sky Interlude – Spring Skies.
How many circumpolar stars appear in your sky depends on where you are. At the Earth’s North and South Poles, every visible star is circumpolar. That is, at Earth’s North Pole, every star north of the celestial equator is circumpolar, while every star south of the celestial equator stays below the horizon. At the Earth’s South Pole, it’s the exact opposite. Every star south of the celestial equator is circumpolar, whereas every star north of the celestial equator remains beneath the horizon.
At the Earth’s equator, no star is circumpolar because all the stars rise and set daily in that part of the world. You can (theoretically) see every star in the night sky over the course of one year. In practice, of course, things like clouds and horizon haze get in the way.
Places in between the equator and poles have some stars that are circumpolar, some stars that rise and set daily (like the sun), and some stars that remain below the horizon all year round. In short, the closer you are to the North or South Pole, the greater the circle of circumpolar stars; the closer you are to the Earth’s equator, the smaller the circle of circumpolar stars.
We in the Northern Hemisphere are lucky to have a moderately-bright star, Polaris, nearly coinciding with the north celestial pole – the point in the sky that’s at zenith (straight overhead) at the Earth’s North Pole.
Draw an imaginary line straight down from Polaris, the North Star, to the horizon, and presto, you have what it takes to draw out the circle of circumpolar stars in your sky.
In the Northern Hemisphere, an imaginary vertical line from the north celestial pole to your horizon serves as a radius measure for the circle of circumpolar stars in your sky. The closer you are to the Earth’s North Pole, the closer the north celestial pole is to your zenith (overhead point).
For people in the Northern Hemisphere, Polaris nearly pinpoints the center of the great big circle of circumpolar stars on the sky’s dome; and the imaginary vertical line from Polaris to the horizon depicts the radius measure. (See the above chart, which has this line drawn in for you.) Let your arm serve as a circle compass, enabling you to envision the circle of circumpolar stars with your mind’s eye. Closer to the equator, the circle of circumpolar stars grows smaller; nearer to the North Pole (or South Pole) the circle of circumpolar stars grows larger.
This technique for locating the circle of circumpolar stars works in the Southern Hemisphere, as well. However, it’s trickier to star-hop to the south celestial pole – the point on the sky’s dome that’s at zenith over the Earth’s South Pole. Practiced stargazers in the Southern Hemisphere rely on the Southern Cross, and key stars, to star-hop to the south celestial pole, as depicted in the illustration below:
The Big Dipper and the W-shaped constellation Cassiopeia circle around Polaris, the North Star, in a period of 23 hours and 56 minutes. The Big Dipper is circumpolar at 41 degrees north latitude, and all latitudes farther north.
The Southern Cross is circumpolar anywhere south of 35 degrees south latitude; yet, in the Northern Hemisphere, it’s the W or M-shaped constellation Cassiopeia that’s circumpolar at all places north of 35 degrees north latitude. (Scroll upward to the chart showing Cassiopeia at nightfall for mid-northern latitudes.)
By the way, Cassiopeia lies on the opposite side of Polaris from the Big Dipper. So from mid-northern latitudes, the Big Dipper and Polaris help you to locate Cassiopeia. See the above animation, in which all the stars revolve full circle around the celestial pole each day – or more precisely: every 23 hours and 56 minutes.
If Cassiopeia is circumpolar in your sky, then the Southern Cross never climbs above your horizon; and conversely, if the Southern Cross is circumpolar in your sky, then the constellation Cassiopeia never climbs above the horizon.
As seen from the tropics (and subtropics), neither the Southern Cross nor Cassiopeia is circumpolar. From this part of the world, the Southern Cross rises over the southern horizon when Cassiopeia sinks below the northern horizon; and conversely, Cassiopeia rises over the northern horizon when the Southern Cross sinks below the southern horizon.
Sky wheeling around Polaris, the North Star. Image via Shutterstock.
Bottom line: Circumpolar stars stay above the horizon all hours of the day, every day of the year. Although you can’t see them, they’re up even in daytime.
Around the time of the June solstice, the sun sets at virtually the same time in both New York City, New York, and St. Augustine, Florida. On June 21, 2019, the sun sets around 8:30 p.m. Eastern Daylight Time (EDT) in both places.
What’s going on here? Doesn’t the sun set later farther north at this time of year? What about the phenomenon of midnight twilight and midnight sun, after all? It’s true that – for places farther north in summer – the sun stays out longer. But St. Augustine lodges about 7.5 degrees of longitude to the west of New York City. Our planet takes about 30 minutes to rotate this 7.5 degrees.
Therefore, on any day of the year, the sun reaches its noontime position some 30 minutes later in St. Augustine than it does in New York City. For instance, on June 21, 2018, the noonday sun reaches its high point for the day at 12:58 p.m. EDT in New York City – yet in St. Augustine, solar noon comes about 30 minutes later, at 1:27 p.m. EDT.
Because New York is appreciably north of St. Augustine, New York’s afternoon daylight (from solar noon to sunset) lasts some 30 minutes longer on the day of the June solstice than it does in St. Augustine.
Thus, the longer period of daylight in New York cancels out the later noontime appearance of the sun in St. Augustine, to give both localities the same sunset time on the day of the June solstice. The table below helps to clarify.
NYC and St. Augustine both use Eastern time. But the noonday sun comes 30 minutes later to St. Augustine because it resides 7.5 degrees of longitude west of New York City.
In other words, from sunrise to sunset on the June solstice, New York City has about an hour more daylight than St. Augustine does. (That’s 30 minutes more morning daylight and 30 minutes more afternoon daylight.) Although the sunset occurs at virtually the same time for both cities, the sunrise happens an hour earlier in New York City. Look again at the sunrise/solar noon/sunset table above.
Earth’s terminator – line of sunset – nearly parallels the Eastern Seaboard on the day of the June solstice.
The image above is a simulated view of Earth as the sun is setting around the time of the June summer solstice. Note that the terminator pretty much aligns with the U. S. East Coast, providing a similar sunset time for coastal dwellers.
Enter the equinoxes
Some three – and nine – months after the June solstice, St. Augustine and New York City receive the same amount of daylight on the days of the September and March equinoxes. On the equinoxes, noontime as well as sunrise and sunset come 30 minutes later in St. Augustine than they do in New York City. The simulated view of Earth below shows the terminator – the sunrise line – running due north and south on the equinox. Neither the sunrise terminator nor sunset terminator comes anywhere close to aligning with the U.S. East Coast at either equinox.
The terminator – sunrise line – runs due north and south on the equinoxes. The sunset line, though not shown, also runs north and south. Image via Earth and Moon Viewer.
Sunrise/solar noon/sunset times on March 20, 2020
Enter the December solstice
Six months after the June solstice, it’s the December winter solstice for the Northern Hemisphere, coming yearly on or near December 21. Now, the situation is reversed from the June solstice, with St. Augustine receiving an hour more daylight than New York City.
Because St. Augustine lies appreciably south of New York City, St. Augustine’s morning daylight (from sunrise to solar noon) lasts 30 minutes longer than in New York City on the day of the December winter solstice. Thus, the more daylight in St. Augustine cancels out the earlier noontime in New York City, to give both localities the same sunrise time on the December solstice. (See sunrise/solar noon/sunset table below.)
Simulation of the line of sunrise as it hits the U.S. eastern seaboard around the December solstice. Image via U.S. Naval Observatory.
Look above at the simulated view of Earth as the sun is rising over the Eastern Seaboard of the United States on the day of the winter solstice. Note that the terminator – the sunrise line – pretty much coincides with the East Coast, giving a similar sunrise time for residents along the Atlantic Seaboard.
From sunrise to sunset on the day of the winter solstice, St. Augustine residents enjoy about an hour more daylight than those in New York City. Although the sunrise occurs at about the same time for both cities, the sunset happens an hour later in St. Augustine on the day of the winter solstice.
Bottom Line: On the day of the June summer solstice, the sun sets at the same time in both St. Augustine, Florida, and New York City, New York. However, New York City enjoys an hour more daylight. Six months later, on the day of the December solstice, it’s the exact opposite. It’s the sunrise that happens at the same time in both places, yet it’s then St. Augustine’s turn to enjoy an extra hour of sunshine.
With its thin atmosphere and low gravity, Mars offers unique challenges for those who want to fly an aircraft there. But – as part of its Mars 2020 mission – NASA has developed a technology demonstration for a heavier-than-air vehicle on the red planet. NASA said on June 6, 2019, that its Mars Helicopter flight demonstration project has now successfully passed a number of key tests. It said:
The small, autonomous helicopter will be the first vehicle in history to attempt to establish the viability of heavier-than-air vehicles flying on another planet.
That first test flight above the surface of Mars is scheduled for 2021. MiMi Aung, project manager for the Mars Helicopter at NASA’s Jet Propulsion Laboratory in Pasadena, California, said:
Nobody’s built a Mars Helicopter before, so we are continuously entering new territory. Our flight model – the actual vehicle that will travel to Mars – has recently passed several important tests.
View larger. | This image of the flight model of NASA’s Mars Helicopter was taken on Feb. 14, 2019, in a cleanroom at NASA’s Jet Propulsion Laboratory in Pasadena, California. The aluminum base plate, side posts, and crossbeam around the helicopter protect the helicopter’s landing legs and the attachment points that will hold it to the belly of the Mars 2020 rover. Image via NASA/JPL-Caltech.
Back in January 2019 the team operated the flight model in a simulated Martian environment. Then the helicopter was moved to Lockheed Martin Space in Denver for compatibility testing with the Mars Helicopter Delivery System, which will hold the 4-pound (1.8-kilogram) spacecraft against the belly of the Mars 2020 rover during launch and interplanetary cruise before deploying it onto the surface of Mars after landing.
As a technology demonstrator, the Mars Helicopter carries no science instruments. Its purpose is to confirm that powered flight in the tenuous Martian atmosphere (which has 1 percent the density of Earth’s) is possible and that it can be controlled from Earth over large interplanetary distances. But the helicopter also carries a camera capable of providing high-resolution color images to further demonstrate the vehicle’s potential for documenting the red planet.
Future Mars missions could enlist second-generation helicopters to add an aerial dimension to their explorations. They could investigate previously unvisited or difficult-to-reach destinations such as cliffs, caves and deep craters, act as scouts for human crews or carry small payloads from one location to another. But before any of that happens, a test vehicle has to prove it is possible.
We expect to complete our final tests and refinements and deliver the helicopter to the High Bay 1 clean room for integration with the rover sometime this summer, but we will never really be done with testing the helicopter until we fly at Mars.
In other news about Mars 2020, NASA said on June 14, 2019, that its engineers have now successfully attached the remote sensing mast to the Mars 2020 rover. The engineers were excited and happy about it, as you can see from the image below.
View larger. | Engineers working on NASA’s Mars 2020 project take a moment after attaching the remote sensing mast to the Mars 2020 rover. This image was taken on June 5, 2019, in the Spacecraft Assembly Facility’s High Bay 1 clean room at NASA’s Jet Propulsion Laboratory in Pasadena, California. Read more about this image.
The Mars 2020 rover – with the Mars Helicopter test – will launch on a United Launch Alliance Atlas V rocket in July 2020 from Space Launch Complex 41 at Cape Canaveral Air Force Station, Florida. When the mission lands in Mars’ Jezero Crater on February 18, 2021, the 2020 rover will conduct geological assessments of its landing site on Mars, determine the habitability of the environment, search for signs of ancient Martian life and assess natural resources and hazards for future human explorers. The Mars Helicopter will be a step foward in heavier-than-air travel above the surface of Mars. Click here more information about Mars 2020.
Bottom line: NASA’s Mars 2020 mission is making great progress. The Mars Helicopter project has passed some key tests. Engineers at NASA’s Jet Propulsion Laboratory have now attached the Mars 2020 rover’s remote sensing mast.