Night Sky with the Naked Eye
How to Find Planets, Constellations, Satellites and Other Night Sky Wonders without a Telescope
By Bob King
Page Street Publishing Co. Copyright © 2016 Bob King
All rights reserved.
Wave "Hi!" to the Astronauts
Curious about what's flying overhead at night? Learn when and where to see the International Space Station, Iridiums and other bright satellites. We explore the role of Earth's shadow in satellite visibility and discover the surprising reason why astronauts float in space.
Go out on the next clear night and look for Earth's shadow (here).
Find the compass directions at night no matter where you are (here).
Use online resources and phone apps to find out when the space station will pass over your house (here).
Catch an Iridium flare (here).
Use your camera to take a photo of the space station (here).
Has this happened to you? You step outside on a summer night to soak in the beauty of the stars, and out of the corner of your eye, one of them takes off across the sky as if unpinned from the firmament. Chances are you're not looking at a star at all, but something much closer to home fashioned out of aluminum, titanium and carbon fiber. An artificial satellite.
Satellites come into view when the sun is below the horizon for an observer on the ground — ordinarily at dusk or dawn — but still shines high overhead where the satellite orbits. Evening and morning twilight are the best times for satellite-watching, especially during the summer months, when twilights last half the night. Since satellites orbit 100 or more miles (161 km) high, they're able to "poke their heads" into the sunshine the same way a mountain peak catches sunlight long after the sun has set in the valley below. Seen against the darkening sky, a sunlit satellite looks like a bright, moving star.
Satellites orbit dozens of times higher than Mt. Everest (generally between 250 and 500 miles [400 and 800 km]) and remain in sunlight for a couple of hours after sunset and before sunrise. The International Space Station and most science satellites operate in low Earth orbit (or LEO) between 112 and 1,242 miles (180 and 2,000 km) high. Navigation satellites, like the familiar GPS network our smartphones use to help us find a destination, orbit between 1,250 and 22,232 miles (2,000 and 35,780 km). Weather satellites eye the globe from high Earth orbit at 22,232 miles (35,780 km) and beam down the images we see on the nightly TV weather.
The reason we see far more during twilight instead of late at night has to do with Earth's shadow. On a sunny day, trees and buildings cast shadows on the ground, and so does the entire planet. Earth's shadow projects up through the atmosphere and into outer space. While you and I find ourselves in Earth's shadow immediately after sunset, satellites, at their much higher altitude, escape it for a time until they're eclipsed from view as shown in the diagram.
ACTIVITY: Amazingly, you can see Earth's shadow any clear evening or morning. Face east starting about 10 minutes after sunset (or west about 30 minutes before sunrise) and look for a purplish-gray band with a fuzzy edge all along the horizon directly opposite the sun. A delicate rosy glow called the Belt of Venus caps the shadow along its length. Although we might never know who first coined the term, it likely refers to the alluring, magical band worn by that famous Roman goddess. Also known as the anti-twilight arch, it's formed when reddened sunlight is scattered back to our eyes by air higher up in the atmosphere still illuminated by the sun.
During twilight, the sun has set for observers on the ground and Earth's shadow hovers low in the eastern sky. But as night progresses, the shadow looms higher and higher, covering more and more sky. One by one, depending on their altitude, satellites are eclipsed from view the same way you'd walk from sunlight into the shadow of a building.
At dawn, favorable conditions return as the shadow lumbers off toward the western horizon. Summer months are better than winter for satellite viewing; not only are twilights longer, but because of a favorable sun angle (the sun never gets very far below the northern horizon even at midnight), part of the sky remains shadow-free all night. Winter's just the opposite. The sun is so far below the horizon that the shadow Earth casts covers virtually the entire sky for much of the night.
Sometimes a satellite will begin its flight in sunlight and then enter Earth's shadow partway through a pass. I think you can guess what happens next. Cut off from sunlight, the satellite quickly fades from view in mid-flight. You're most likely to notice this with the space station because it's bright to begin with, making its disappearance that much more dramatic.
Speaking of which, you're probably curious about what's flying over your head tonight. The brightest and easiest satellite by far is the International Space Station (ISS), which orbits the Earth once every 92 minutes. Knowing when and where to look and then seeing it arrive exactly on time over your house remains one of the greatest skywatching thrills.
During a typical pass, the station glares as brightly as the planet Jupiter and at times even rivals the brilliance of Venus. That shiny brightness makes it look huge to the eye through an effect called irradiation, where light from a bright source seen against a dark background spreads out on our retinas. We can't see the true shape or size of the ISS except through powerful binoculars or a small telescope. Although satellites range in size from a small loaf of bread to the football field-long space station, all appear star-like to the eye because they're hundreds of miles away.
ACTIVITY: Ready to find the station? First off, you need to know your directions, and don't worry if you lack a compass. Just face in the general direction of sunset — that's west. Now stick out your right arm. That's north. Your left arm points south and your back faces east. If you're in an unfamiliar place and unsure of the sunset direction, launch the compass app on your mobile phone. For an iPhone, tap the compass icon in the Utilities folder. Android users can download a free compass app (here).
No phone? No worries. The Big Dipper has your back.
Look around the sky to find the Dipper's familiar bucket-and-handle shape, then draw a line through the two stars at the end of bucket and extend it out about 5 times, and you'll arrive at Polaris, the North Star. Polaris looks just like a Dipper star and is located nearly due north. Face Polaris and south will be at your back, west to your left and east to your right. See the diagram here for more details.
Next, you'll need the station's arrival time and flight path. There are lots of ways to find this information. You can jump online to Spot the Station (spotthestation.nasa.gov) or Spaceweather (spaceweather.com/flybys/), key in your location and get a table of dates, rise and transit times (when the ISS is highest), magnitudes (brightness) and where to look.
Prefer a more visual approach? Go to Heavens Above (www.heavensabove.com), log in and select your city from a list or map. Under the Satellites heading, choose ISS and you'll be directed to a 10-day table of visibility times. Notice that the times are given on the 24-hour clock where 6 a.m. is 6:00 and 6 p.m. is 18:00. The site provides the same basic information as Spaceweather and others — brightness, start direction, highest point and end direction — plus a map!
The map shows the satellite's path with time ticks along its length, letting you see at a glance the time and direction of travel. Because the map is a two dimensional representation of the domed sky above, map center is the overhead point or zenith. North's at top, south at bottom, west to the right and east to the left.
Many tracking sites pinpoint a satellite's location using altitute (alt), its height above the horizon, and azimuth (az) or compass direction. Altitude is measured in degrees with the horizon at 0° and the overhead point at 90°. Azimuth goes clockwise around the horizon starting at 0° (or 360°) for north. East is 90°, south is 180° and west is 270°.
If you have a mobile phone, you can skip checking the website and instead download a free ISS app, several of which are listed at the end of this chapter (here). Once you activate the app and it locks onto your location, you'll get daily predictions and paths at the touch of a button. In-app options, available for a small fee, let you track the Hubble Space Telescope, China's Tiangong Space Station, current comets and more. We do live in wonderful times.
Assuming the ISS will be passing over your neighborhood tonight, let's head outside. Give yourself a few minutes beforehand to get oriented and allow your eyes to get used to the dark. Now for the fun part — watching the astronaut crew fly overhead, bundled inside a brilliant, pale yellow "star." I'm always amazed at the station's precise timing. You expect it at 8:02 p.m. and sure enough, a bright, moving "star" appears in the northwestern sky at the appointed minute. It may be science, but it sure feels like magic.
During a typical pass the ISS makes its first appearance low in the western sky, tracks across the northern or southern sky and then drops off into the east. Whether you live in the suburbs, a small city or the countryside, you'll have no problem seeing the station because it shines so brightly. And unlike an airplane, its light stays steady, neither blinking nor flashing.
Planes have red and green wing lights that flash at regular intervals, while the ISS and most other satellites shine with a steady light like an ordinary star. Occasionally, an inoperable satellite or the rocket stage used to launch it into orbit tumbles out of control, directing random flashes of sunlight toward the ground as it spins. But the rhythm's all wrong and the glints are white, so it's easy to tell the two apart after a few moments' observation. Likewise for meteors, which streak by in a flash and disappear.
As the station flies over, study its color. Most satellites are colorless, but the ISS has eight large solar arrays made with a gold-colored material called kapton, lending it a pale yellow hue. Every so often, sunlight will reflect off the solar panels in your direction and cause the ISS to temporarily flare in brightness. Keep an eye out for these welcome surprises.
The ISS travels slowly and shines modestly when it first crests the western horizon because it's still far off in the distance, but by the time it's nearly overhead, only 250 miles (400 km) separate you and the astronauts on board. Not only does the craft shine brightest then, but it will appear to move fastest, comparable to an airplane. Don't forget to wave "Hi!" as the human crew zooms by at more than 17,000 miles (27,360 km) an hour. Funny, that's probably as close as most of us ever come to meeting an astronaut.
Assuming the ISS is visible at dusk or dawn, you'll see one to two passes spaced an hour and a half apart. One of them is usually cut short when the station gets eclipsed by Earth's shadow. From the point of view of the astronauts, the sun is setting. Just like sunsets on the ground, where the landscape glows in reds and oranges, the space station takes on sunset colors. I've tried without success to see these color changes with the naked eye, but it's surprisingly easy in binoculars. Give it a try.
Every few months, the Russian Soyuz (means "Union") spacecraft ferries astronauts and cosmonauts to and from the space station. In the future, private launch facilities in the U.S. will contract with NASA to deliver crews. Cargo ships also routinely dock with the station to deliver food, fuel, parts and more. Their comings and goings make for great observational fun. On approach, a cargo ship will inch closer and closer to the ISS during repeated orbits until the two pass over your head in tandem, like a classic game of cat and mouse.
After spying your first few ISS passes, you'll soon notice its arc across the sky can vary from low to overhead. Depending on where your city is in relation to the space station's orbital location when it pops into the sky, it might beat a path across the north, cut a diagonal from northwest to southeast or barely crest the southern horizon.
Curious why it always travels from west to east and never the other way around? Most satellites are launched to the east to take advantage of the free speed boost they get from our planet's west-to-east rotation. A satellite taking off from Cape Canaveral gains 915 miles (1,473 km) an hour because Earth rotates at that speed at that latitude. Who doesn't love a freebie? Thanks to the spinning Earth, the rockets that launch satellites can be smaller and carry less fuel, saving money.
The space station orbits Earth at an angle or inclination of 51.6° to the equator, a steep tilt. Its inclination defines where on Earth you can see it. The ISS will pass overhead for anyone living between latitudes 51.6° south and 51.6° north. It can be seen for some distance beyond those limits lower in the sky but not at the overhead point. Most of humanity lives within or near this latitudinal zone, so about 95 percent of the people on Earth have the opportunity to view the space station.
That's not true for the Hubble Space Telescope, which has an orbital inclination of 28.5°. I attended a conference once in Charleston, South Carolina (latitude 33° north) and made sure to watch for the Hubble, which is never visible from my home in northern Minnesota (latitude 47° north). Neither as bright nor as big as the space station — Hubble's about the size of a large school bus — it orbits 342 miles (550 km) up and loops around Earth once every 96 minutes. Check it out if you live anywhere from the southern U.S. to central South America; there's a link for it in Heavens Above.
Zipping around the planet at 17,000 mph (27,360 kph) or 92 minutes per orbit, the International Space Station crew experiences 15–16 sunrises and sunsets every single day. By all accounts, they never tire of looking at the Earth with its ever-changing clouds and swift sunrises. The steep pitch of the space station's orbit takes the crew right up near the auroral zone, guaranteeing regular sightings of both the northern and southern lights for all on board.
Astronauts Earth-gaze in their free time but otherwise have to go to work like us earthbound folks. The ISS serves as a space laboratory, where the crew conducts experiments in biology, physics, meteorology, astronomy and chemistry in a unique environment called microgravity.
Seeing astronauts floating around the cabin and inside the cupola, a room with a picture window view of Earth, you'd think there's no gravity up there. Nothing could be further from the truth. Assume for a moment there isn't. Can you guess what would happen? The entire station would drift away from Earth and take up an orbit around the sun. Earth's gravity acts as a tether to keep it from floating away. So what causes weightlessness or the appearance of zero G? If you've ever been in an elevator that descended to a lower floor faster than expected, you may have felt for a moment as if you were going to float right off the ground. The same unsettling experience can happen during severe air turbulence when the plane you're flying in feels as if it's dropping out from under you. Now imagine the space station as an elevator.
Every minute of every orbit, the space station is falling, falling, falling to Earth. It doesn't crash because it's moving forward in its orbit fast enough to make it around the curve of the globe and continue along its path. On a sphere, the surface is always dropping away beneath you as you circle about it; if the Earth were flat, the ISS would crash after its first orbit. In case you were in doubt, the daily tweets from happy astronauts provide yet another proof that Earth is a sphere!
Don't expect to see the International Space Station every night. Due to the perturbing effects of Earth's gravity, the ISS goes through cycles called "windows of visibility," each of which lasts about two months. During that time, it first appears in the dawn sky for about a month followed by a similar stint in the evening sky and then disappears altogether for a time. Oh, it's still there, zipping along as always, but in the daytime sky. Soon enough, the station will reappear at dawn to begin a new go-round.
The cycle repeats throughout the year with one exception: In late May–early June, on the eve of northern hemisphere summer, the station's orbit and Earth's day-night terminator — the line separating day from night — nearly align. From the astronauts' point of view, the sun never sets. Back on the ground, between latitudes 40° and 55° north (or 40° and 55° south during southern hemisphere summer), the ISS remains in sunlight one orbit after another without ever dipping into Earth's shadow. Instead of one or two passes a night, you can watch up to six from dusk to dawn. Those who manage to pull this off by staying out all night can proudly boast they took part in a successful ISS marathon. Southern hemisphere skywatchers have their marathon in December. (Continues...)
Excerpted from Night Sky with the Naked Eye by Bob King. Copyright © 2016 Bob King. Excerpted by permission of Page Street Publishing Co..
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