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.
This month spot the Moon together with Mars and Venus, along with the flickering star Fomalhaut, which had itself a planet…until it didn’t! Check out the video below produced by NASA’s Jet Propulsion Laboratory to learn more.
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!
NASA’s Mars 2020 mission is planned to launch this summer, sending the Perseverance rover and the first-ever Mars helicopter to the Red Planet. So, if you’re a Mars exploration fan, this month is a great time to spot Mars yourself. Check out the video below produced by NASA Jet Propulsion Laboratory to learn more.
The sky will put on a show Nov. 11 when Mercury journeys across the Sun. The event, known as a transit, occurs when Mercury passes directly between Earth and the Sun. From our perspective on Earth, Mercury will look like a tiny black dot gliding across the Sun’s face. This only happens about 13 times a century, so it’s a rare event that skywatchers won’t want to miss! Mercury’s last transit was in 2016. The next won’t happen again until 2032!
“Viewing transits and eclipses provide opportunities to engage the public, to encourage one and all to experience the wonders of the universe and to appreciate how precisely science and mathematics can predict celestial events,” said Mitzi Adams, a solar scientist in the Heliophysics and Planetary Science Branch at NASA’s Marshall Space Flight Center in Huntsville, Alabama. “Of course, safely viewing the Sun is one of my favorite things to do.”
This year’s transit will be widely visible from most of Earth, including the Americas, the Atlantic and Pacific Oceans, New Zealand, Europe, Africa, and western Asia. It starts at about 6:35 a.m. CST, but viewers in some areas, such as the West Coast, will have to wait until the Sun rises at their location to see the transit already in progress. Thankfully, this transit will last almost six hours, so there will be plenty of time to catch the show. At about 9:20 a.m. CST, Mercury’s center will be as close as it is going to get to the Sun’s.
Mercury’s tiny disk, jet black and perfectly round, covers a tiny fraction of the Sun’s blinding surface — only 1/283 of the Sun’s apparent diameter. So you’ll need the magnification of a telescope (minimum of 50x) with a solar filter to view the transit. Never look at the Sun directly or through a telescope without proper protection. It can lead to serious and permanent vision damage. Always use a safe Sun filter to protect your eyes!
Scientists have been using transits for hundreds of years to study the way planets and stars move in space. Edmund Halley used a transit of Venus in 1761 and 1769 to determine the absolute distance to the Sun. Another use of transits is the dimming of Sun or star light as a planet crosses in front of it. This technique is one way planets circling other stars can be found. Scientists can measure brightness dips from these other stars (or from the Sun) to calculate sizes of planets, how far away the planets are from their stars, and even get hints of what they’re made of.
The International Observe the Moon Night is happening Oct. 5!
In the wake of the beautiful Harvest Moon seen Sept. 14, Earth’s satellite now enters its waning phase, shrinking slice by slice into a visible semicircle as the rotating Earth spins around the Sun and its shadow is cast past us onto the Moon.
This period of waxing or waning is commonly known as a “gibbous” period, a term meaning “convex” or “rounded.” The term originated in the 15th century, though the Oxford English Dictionary suggests it was first applied to describe the Moon in 1690.
Gradually, over the coming month, the Moon will cycle toward its next full period – the Hunter’s Moon, also known as the Blood Moon or Sanguine Moon, due in mid-October. Various Native American tribes gave the Moon this name to reflect the falling leaves of autumn and the fattening of deer and other animals to prepare for the winter to come.
Speaking of coming events, the International Observe the Moon Night is happening Oct. 5! Learn more about NASA’s plans and how you can join in the fun here.
Take a moment this weekend to gaze at the beautiful Harvest Moon! The Moon will appear full Thursday night through Sunday morning. Early Saturday morning, Sept. 14, will mark the actual full Moon.
The Harvest Moon gets its name from agriculture. In the days before electric lights, farmers across the Northern Hemisphere depended on bright moonlight to extend the workday beyond sunset. It was the only way they could gather their ripening crops in time for market. The full Moon closest to the autumnal equinox became “the Harvest Moon,” and it was always a welcome sight.
The term became further entrenched in popular culture thanks to a 1903 pop tune called “Shine on Harvest Moon.”
A Harvest Moon inflated by the moon illusion — a common phenomenon with full moons seen close to the horizon, where familiar Earth objects such as trees or buildings lend the Moon a false sense of size — is simply beautiful to view, but even more so to the farmers gathering their crops in on those cool autumn evenings.
35.8 million miles is definitely not what most of us would consider “close.” But in planetary terms, close is definitely relative! On July 31, Mars will be 35.8 million miles from Earth, which is the closest it has been to Earth in 15 years. What does this mean for sky watchers? It means the Red Planet will appear super bright, and with its orange-red color, will be hard to miss in the nighttime sky. From July 27-30, the point in Mars’ orbit will come closest to Earth, and will be closest to Earth before sunrise Eastern Time on July 31.
What defines a “close approach?” The minimum distance from the Earth to Mars is about 33.9 million miles and does not happen very often. Because Earth and Mars have elliptical orbits and are slightly tilted to each other, all close approaches are not equal. When Mars slowly approaches what astronomers call opposition, it and the Sun are on opposite sides of the Earth. Earth and Mars align in opposition about every two years (fun fact: this is why most NASA missions to the Red Planet are at least two years apart – to take advantage of the closer distance). Opposition to Mars is at its closest to the Sun every 15 to 17 years, when excellent views of the Red Planet from Earth can occur. This is what is happening on the early morning hours of July 31.
Is 35.8 million miles the closest Mars has ever been to Earth? Nope. In 2003, Mars was 34.6 million miles from Earth and the closest it had been in nearly 60,000 years. This type of proximity won’t occur again until 2287. But, there will be another close approach in October 2020 when the distance between the Red Planet and Earth will be 38.6 million miles.
Now that we’ve gotten all of that out of the way, what does this mean for you, the novice astronomer or general sky-watcher? It means that if you have clear skies where you live, go outside on the overnight hours of July 30 or early morning hours of July 31 and look up. The planet will be brighter than usual and will have an orange or red haze. You can also look through a telescope. If weather is bad where you are, NASA will be streaming live from the Griffith Observatory.
There was a very bright green fireball seen by hundreds of eyewitnesses surrounding Lake Michigan early this morning at 1:25:13 AM Central Time (February 6, 2017). The reports from these individuals and the video information from dash cameras and other cameras in the region indicate that the meteor originated 62 miles above West Bend, Wisconsin and moved northeast at about 38,000 miles per hour. It disrupted about 21 miles above Lake Michigan, approximately 9 miles east of the town of Newton. The explosive force of this disruption was recorded on an infrasound station in Manitoba, some 600 miles away – these data put the lower limit energy of the event at about 10 tons of TNT, which means we are dealing with a meteoroid – orbit indicates an asteroidal fragment – weighing at least 600 pounds and 2 feet in diameter. Doppler weather radar picked up fragments (meteorites) falling into Lake Michigan near the end point of the trajectory.
Ground track and Doppler radar signature (done by Marc Fries at NASA Johnson Space Center); an animation of the orbit and approach of the meteoroid is being prepared and should be available soon. We will continue to look at data as it comes in and revise the calculations if necessary.