Happy equinox, Earthlings! Sept. 22 marks the fall equinox, when day and night are nearly equal.
“However, that day/night length depends on where you are on Earth,” said NASA solar scientist Mitzi Adams. “For example, at the North and South Poles, the length of the day and night is six months!”
At the North Pole, the Sun will sink below the horizon for a kind of twilight from now until sometime in October when it will be completely dark, explained Adams. Spring twilight begins a few weeks before the vernal, or spring, equinox in March, when the Sun rises above the horizon again.
This only happens twice in Earth’s year-long trip around the Sun. The rest of the year, the Sun shines unevenly over the Northern and Southern Hemispheres. That’s because Earth’s axis is tilted with respect to the Sun-Earth plane. But on these special days – the spring and fall equinox – the Sun shines equally on both north and south.
Here in the Northern Hemisphere, it’s the first day of astronomical fall. From now until the beginning of spring, nighttime hours will last longer than daylight as the Sun travels a shorter arc across the sky each day. The Sun has its shortest path of the year at the time of the winter solstice — the shortest day and longest night of the year — when sunrise and sunset are as far south as they can go (at any one location). It’s just the opposite in the Southern Hemisphere, where September 22 kicks off astronomical spring.
The equinox—meaning “equal night” in Latin—occurs at 8:31 a.m. CDT.
For Comet Vocabulary, please read to the end of the post.
For most, early July is when most people living in the United States look to the skies to watch dazzling firework shows. However, this month there is a different kind of show happening in the sky.
Comet Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) was only discovered a few months ago on March 27 by NASA’s NEOWISE telescope and has quickly become a popular solar system visitor. Its popularity is warranted, however, as it is the brightest comet since Comet Hale-Bopp that passed by Earth 23 years ago in 1997.
Comet nuclei are cosmic snowballs of frozen gases, rock and dust that orbit the sun. They can range in size from a few miles to tens of miles wide, and the nucleus of NEOWISE measures about 3 miles across. When these comets approach the sun, their frozen bodies start to sublimate, and they spew dust and gasses in a tail that can span millions of miles.
Comet NEOWISE made its harrowing close approach to the sun, known as its perihelion, on July 3, and it is now zooming past the Earth on its way back out of the solar system. NEOWISE will make its closest approach (64 million miles) to Earth on July 22, but the best viewing window is happening right now until July 19.
NEOWISE can be seen with the naked eye, but for an even better viewing experience, binoculars or even a telescope is recommended. As for which to choose, binoculars are your current best option. “Definitely use binoculars for now – the tail of NEOWISE is at least 7 degrees long, which is much bigger than the field of view of most telescopes,” said Bill Cooke, lead of NASA’s Meteoroid Environment Office at Marshall Space Flight Center. “Binoculars will allow you to see the whole thing, whereas a telescope only shows a tiny part.”
To see NEOWISE, start looking in the northwestern sky about an hour after sunset. The comet will be below the stars that make up the bowl of the Big Dipper and shining nearly as brightly at a magnitude 3. If you are an early riser, you can still see NEOWISE about an hour before sunrise in the northeastern horizon until the end of the week.
You need a clear view of the horizon to see this comet. Beaches, fields, and areas with higher elevations are all great observation spots. In areas with more light pollution, binoculars may be necessary for viewing. This is definitely a once-in-a-lifetime event, as NEOWISE won’t be visiting again for 6,800 years!
By: Bill Cooke
Lead, NASA Meteoroid Environment Office
The media is currently broadcasting the prediction of an outburst of the alpha Monocerotid meteor shower on the night of November 21. The researchers making the prediction, Dr. Peter Jenniskens and Esko Lyytinen, have made calculations that indicate that there may be zenithal hourly rates as high as 400 to 1000 meteors per hour around 11:50 PM Eastern Standard Time (10:50 PM Central and 9:50 PM Mountain; you will note that I am not giving a Pacific time – more on that later). These are impressive numbers, generating lots of buzz in the media. I love meteor outbursts and storms, so I was initially quite excited – I mean, what’s there not to like about an impromptu display involving lots of meteors from a yet-to-be discovered comet?
But as the media inquiries increased, I began to wonder if all the attention is justified. Being a meteor shower forecaster, I am all too aware of the fact that such predictions (including mine), while pretty accurate on the timing, often estimate a shower intensity higher (factors of a few) than what actually takes place. So I decided to take a more detailed look, starting with some dumpster diving for old papers about this shower and making a few calculations of my own. That’s when the skepticism kicked in – I now think there is a pretty good chance there may be no outburst at all. And even if there is, it won’t be as impressive as many think. Allow me to share…
In Dr. Jenniskens and Lyytinen’s work, the Earth is forced to pass through the center of alpha Monocerotid meteor stream (AMOs for short) during the shower’s 1925 and 1935 outbursts. We have no idea if this actually happened, but it is a reasonable assumption if these outbursts were more intense than the last one in 1995. Based on this, they conclude that the AMOs are produced by a long period comet that takes about 500 years to orbit the Sun. IF this is right, then we should pass very close to the center of the meteor stream this year, missing it by a scant 15,000 miles. That’s just a tad closer than we got back in 1995, when the observed zenithal hourly rate was about 400 per hour. And it’s why the forecast rate is so high – closer means the same intensity or better.
However, the intensity of the outburst is very dependent on the size of the parent comet’s orbit. If it is much smaller, or larger, the distance from the stream center will be bigger, and there will not be any sky show, just the normal AMOs, puttering along with their normal rate of 3 or so meteors per hour. And since we have not yet discovered this mysterious parent comet, who knows how close the estimate of the orbit is to the actual? A good reason to step outside Thursday night, because the cool thing is that if an outburst does occur, we will have a pretty good idea of the orbit of this comet – not from observing the comet with telescopes, but by counting its debris as they burn up in our atmosphere.
The old papers I dug up also proved enlightening. I could find no meteor rate numbers for the 1925 outburst – just that it was short, with a fair number of meteors. The 1935 AMO outburst was observed in 2 places – a meteor observer in Begumpet, India and the commanding officer of a U.S. ship in the Philippines each reported seeing a total of just over 100 meteors in a 40 minute span of time. That’s nice, but it certainly is nowhere close to the spectacular rates produced by the Leonid and Draconid meteor storms of the 1900’s. A moderate outburst, yes, but not a meteor storm. Even fewer were seen in 1985, when one observer reported 36 meteors seen over 16 minutes of time. It is true that the calculated rates were in the hundreds per hour, but what matters to the average person is the total number of meteors they will see. Zenithal hourly rates give the theoretical rates for a perfect observer under perfect skies with the shower radiant straight overhead (something that never happens in reality), and while they may be a good way to scientifically measure meteor shower activity, they are poor indicators of what will actually be seen. The observer reports, however, do tell us what we might expect.
And then we come to 1995, the best-observed AMO outburst. Quite a few observers in Western Europe saw about 100 meteors over an hour’s time, consistent with the observations of the previous AMO outbursts. These data do not indicate that we were closer to the AMO stream center in 1925 and 1935, as Jenniskens and Lyytinen suggest; in fact, it appears that AMO outbursts are fairly constant with regard to numbers, with about 100 meteors seen over the less-than-an-hour duration of the outburst. At face value, this would mean no outburst. However, the numbers seeming to be not strongly dependent on distance is possibly good news; even if the researchers’ distance assumptions are wrong, we still may have a chance of a respectable, albeit short, outburst, provided Earth gets “close enough” to the stream center.
At the beginning of this post, I gave times for the predicted peak in the Eastern, Central, and Mountain time zones, but left out Pacific. That’s because the AMO radiant – the point in the constellation of Monocerotis from which the meteors appear to originate – is below the horizon at the peak time for locations west of Denver. That means people on the Pacific Coast will not see this outburst, even if their skies are clear. So if you live there and want to experience the shower, you need to go quite a bit east. If you do, please don’t blame me if the outburst is a no show; as I said, I am a bit skeptical. For the eastern United States, the radiant is not very high in the sky at the forecast peak time (about 23° in Orlando), which is unfortunate since the observed number of meteors is tied to the radiant altitude. The higher the radiant, the more meteors people see. So my computer savvy colleagues have generated this map, which shows the total numbers of meteors you can expect to see if the outburst is similar to that of 1995. Blue is good, red is worse, white means no meteors at all. The decrease in total expected meteors is pretty obvious as you move west
And of course, there is the weather. Remember, you need clear, dark skies to see meteors, and it looks like Mother Nature is going to be mean, with clouds forecast over much of the part of the U.S. that has a chance of observing the outburst. So, if you are gifted with good seeing, give yourself about 45 minutes to adjust to the dark – go out about 10:35 PM Eastern, 9:35 PM Central, or 8:35 PM Mountain. Lie flat on your back, look straight up, and enjoy looking at the night sky (maybe listen to some appropriate tunes, but don’t look at your cell phone, as the bright screen will ruin your night vision). If Jenniskens and Lyytinen are right, you might see some pieces of a comet that awaits discovery, burning up in the atmosphere 60 miles above your head.
That’s worth a couple of hours, I think. Even if there is no outburst, it doesn’t hurt to get out under the stars for a bit.
Well over 100 people in California, Nevada, Arizona and Oregon observed a fireball at 5:35 p.m. PST Dec. 19. This event was unusual not for the brightness of the fireball—similar to that of a crescent Moon—but for the persistent train left behind after the object ablated. This persistent train lasted for minutes (compared to the one second duration of the fireball) and was caused by sunlight reflecting off dust particles left behind by the meteoroid as it broke apart in Earth’s atmosphere. Upper atmosphere winds distorted the train over time, giving it a curvy, “corkscrew” appearance.
An analysis of the eyewitness accounts indicates that the meteor first became visible at an altitude of 48 miles over the Pacific Ocean some 50 miles west of the entrance to San Francisco Bay. Moving west of south at 63,000 miles per hour, it managed to survive only a second or so before ablating and breaking apart at an altitude of 34 miles above the ocean.
A bright fireball lit up skies over Michigan at 8:08 p.m. EST on Jan. 16, an event that was witnessed and reported by hundreds of observers, many who captured video of the bright flash.
Based on the latest data, the extremely bright streak of light in the sky was caused by a six-foot-wide space rock — a small asteroid. It entered Earth’s atmosphere somewhere over southeast Michigan at an estimated 36,000 mph and exploded in the sky with the force of about 10 tons of TNT. The blast wave felt at ground level was equivalent to a 2.0 magnitude earthquake.
The fireball was so bright that it was seen through clouds by our meteor camera located at Oberlin college in Ohio, about 120 miles away.
Events this size aren’t much of a concern. For comparison, the blast caused by an asteroid estimated to be around 65 feet across entering over Chelyabinsk, Russia, was equivalent to an explosion of about 500,000 tons of TNT and shattered windows in six towns and cities in 2013. Meteorites produced by fireballs like this have been known to damage house roofs and cars, but there has never been an instance of someone being killed by a falling meteorite in recorded history.
The Earth intercepts around 100 tons of meteoritic material each day, the vast majority are tiny particles a millimeter in diameter or smaller. These particles produce meteors are that are too faint to be seen in the daylight and often go unnoticed at night. Events like the one over Michigan are caused by a much rarer, meter-sized object. About 10 of these are seen over North America per year, and they often produce meteorites.
There are more than 400 eyewitness reports of the Jan. 16 meteor, primarily coming from Michigan. Reports also came from people in nearby states and Ontario, Canada, according to the American Meteor Society. Based on these accounts, we know that the fireball started about 60 miles above Highway 23 north of Brighton and travelled a little north of west towards Howell, breaking apart at an altitude of 15 miles. Doppler weather radar picked up the fragments as they fell through the lower parts of the atmosphere, landing in the fields between the township of Hamburg and Lakeland. One of the unusual things about this meteor is that it followed a nearly straight-down trajectory, with the entry angle being just 21 degrees off vertical. Normally, meteors follow a much more shallow trajectory and have a longer ground track as a result.
NASA’s Short-term Prediction Research and Transition Center reported that a space-based lightning detector called the Geostationary Lightning Mapper — “GLM” for short — observed the bright meteor from its location approximately 22,300 miles above Earth. The SPoRT team helps organizations like the National Weather Service use unique Earth observations to improve short-term forecasts.
GLM is an instrument on NOAA’s GOES-16 spacecraft, one of the nation’s most advanced geostationary weather satellites. Geostationary satellites circle Earth at the same speed our planet is turning, which lets them stay in a fixed position in the sky. In fact, GOES is short for Geostationary Operational Environmental Satellite. GLM detected the bright light from the fireball and located its exact position within minutes. The timely data quickly backed-up eyewitness reports, seismic data, Doppler radar, and infrasound detections of this event.
Much like the nation’s weather satellites help us make decisions that protect people and property on Earth, NASA’s Meteoroid Environment Office watches the skies to understand the meteoroid environment and the risks it poses to astronauts and spacecraft, which do not have the protection of Earth’s atmosphere. We also keep an eye out for bright meteors, so that we can help people understand that “bright light in the night sky.”
On Monday, Aug. 21, for the first time in almost 100 years, all of North America will be treated to an eclipse of the sun. Those in the path of totality, running from Oregon to South Carolina, will experience one of nature’s most awe-inspiring events — a total solar eclipse.
Scientists, researchers and experts from NASA’s Marshall Space Flight Center in Huntsville, Alabama, will mobilize to experience the eclipse and share it with others. They will join participants from across the agency for a multi-hour broadcast, titled Eclipse Across America: Through the Eyes of NASA, to offer unprecedented live video of the celestial event, along with coverage of activities in parks, libraries, stadiums, festivals and museums across the nation, and on social media.
“It’s going to be a spectacular event,” said Marshall Chief Scientist James Spann. “We’ll be sharing our research and work with people and letting them know how to safely view the eclipse, not only at the events in the path of totality, but also worldwide online and on NASA Television. Excited doesn’t begin to describe how our team feels right now. It truly will be breath-taking, and we can’t wait.”
Marshall experts will be located at two of the broadcast’s 15 locations — Hopkinsville, Kentucky, and Austin Peay State University in Clarksville, Tennessee.
The motion of the moon is what causes eclipses, but the dramatic change in sunlight is what makes them so impressive to observers. But what exactly is happening when the moon passes in front of the sun?
The moon is blocking the sun’s light from reaching Earth, but there is more to the situation than just that. Their relative distance to Earth is one of the most important factors.
The sun is about 400 times farther from Earth than the moon and has a diameter about 400 times larger than the moon. As a result, both the sun and moon (near perigee) appear to be the same size in the sky, allowing the moon to perfectly block out the sun and cast a shadow on Earth during a total eclipse.
The shadow we see while in the path of totality is called the umbra, and the shadow of the surrounding partial eclipse is a penumbra. The shadow from an annular eclipse (when the moon appears smaller than the sun during an eclipse, and so a ring of light is visible around it) is called an anteumbra.
The physics of how each type of shadow is formed is difficult to explain but easy to visualize, so before I tell you about them, here is a picture (technically a ray diagram) of what happens during an eclipse:
For a total eclipse, the moon has to block out all of the sun’s light. To put the moon in the best position, imagine that a person on Earth is standing under the exact middle of the moon, the centerline of a total solar eclipse.
In this case, light coming from the middle of the sun is clearly going to be blocked by the moon, since it is directly in the way and visible light cannot penetrate rock. The most difficult light to block will be coming from the top and bottom of the sun.
To figure out whether the light will be blocked, a bit of drawing can help. If the light is coming from the exact bottom of the sun and you are wondering if a person can see the light while under the exact center of the moon, draw a line between where the light starts and the person’s eyes.
Does the moon get in the way of the line? If yes, then the person is experiencing a total solar eclipse. None of the sun’s light can get past the moon, so the sun is fully blocked.
If the answer is no, but the person is still standing under the center of the moon, then they are in an annular eclipse. The moon is in the perfect position to block all of the sun’s light, but it still fails to do so. In this case, it will appear to be a large black circle with a ring of sunlight called an annulus around it.
A partial eclipse is the most difficult to explain, since it has the most variability. All but a sliver of the sun may be blocked, or the moon can barely cover any of the sun. In general though, a partial solar eclipse happens when the moon is not quite directly between the observer and sun, but is still in the way of some sunlight.
You can use the same process for determining whether a person is experiencing a total solar eclipse to figure out if they are in the penumbral shadow of the moon. A slight complication is that the moon is off center, so it matters more where the origin point of the light is.
If the person is standing a little north of the moon’s center, then the line from origin to person should start from the sun’s southernmost point, the bottom, since the northern light is less likely to be blocked due to the moon being a bit more to the south from the person’s perspective.
If any of the sun’s light is blocked by the moon, then the person is experiencing a partial solar eclipse. The limit of this blockage, where only the slightest amount of sunlight is blocked, is the edge of the penumbra shadow.
If the moon is not blocking any light, then the moon may be close to the sun but there is no eclipse happening on that spot of Earth.
Make plans now to stay up late or set the alarm early next week to see a cosmic display of “shooting stars” light up the night sky. Known for it’s fast and bright meteors, the annual Perseid meteor shower is anticipated to be one of the best potential meteor viewing opportunities this year.
The Perseids show up every year in August when Earth ventures through trails of debris left behind by an ancient comet. This year, Earth may be in for a closer encounter than usual with the comet trails that result in meteor shower, setting the stage for a spectacular display.
“Forecasters are predicting a Perseid outburst this year with double normal rates on the night of Aug. 11-12,” said Bill Cooke with NASA’s Meteoroid Environments Office in Huntsville, Alabama. “Under perfect conditions, rates could soar to 200 meteors per hour.”
An outburst is a meteor shower with more meteors than usual. The last Perseid outburst occurred in 2009.
How to Watch the Perseids
The best way to see the Perseids is to go outside between midnight and dawn on the morning of Aug. 12. Allow about 45 minutes for your eyes to adjust to the dark. Lie on your back and look straight up. Increased activity may also be seen on Aug. 12-13.
For stargazers experiencing cloudy or light-polluted skies, a live broadcast of the Perseid meteor shower will be available via Ustream overnight on Aug. 11-12 and Aug. 13-14, beginning at 10 p.m. EDT.
For a few seconds early Thursday, night turned into day as an extremely bright fireball lit the pre-dawn sky over much of Arizona, blinding all-sky meteor cameras as far away as western New Mexico.
Based on the latest data, a small asteroid estimated at 5 feet (1-2 meters) in diameter – with a mass of a few tons and a kinetic energy of approximately half a kiloton – entered Earth’s atmosphere above Arizona just before 4 a.m. local (MST) time. NASA estimates that the asteroid was moving at about 40,200 miles per hour (64,700 kilometers per hour).
We observed a fireball the morning of May 4 around 12:50am EDT, traveling southwest at about 77,000 mph over the Nantahala National Forest on the Tennessee/North Carolina state line. At its brightest point, it rivaled the full moon. According to Dr. Bill Cooke in NASA’s Meteoroid Environment Office at NASA’s Marshall Space Flight Center in Huntsville, Ala. , “The fireball was bright enough to be seen through clouds, which is an attention getter. In Chickamauga, Ga., one would have thought it was a flash of lightning lighting up the clouds beneath.”