How to Tell Whats What
With all the wonderful things to see in the night sky, its sometimes hard to identify objects. Here are some hints to determine whats what:
Satellites and jets - These objects move fast. Low-flying jets usually reveal blinking lights (plus, you can hear them). High-flying jets and satellites can look very similar, though. Usually, satellites travel in recognizable patterns. If youre not sure whether what you saw was a satellite or a jet - wait 90 minutes. Thats how long it takes for most low-orbiting satellites to orbit the Earth. If it was a jet, chances are it will not be back that soon!
Planets - Some planets have a distinct appearance that gives them away. Mars, for example is bright red. Venus is incredibly bright. But sometimes its hard to tell a planet from a star. One way to settle the matter is with a pair of binoculars or a telescope, but if you dont have those handy, gaze at the object for a few minutes. If it doesnt twinkle as much as the stars around it, it could be a planet. Everything we see from the ground is subject to the effects of the atmosphere, but planets dont twinkle like stars because they are much closer, which
makes them "extended objects," meaning that it's possible to see the disk of the planet (at least with a telescope).
Starlight, on the other hand, comes from so far away that it acts as a
point light source. When the atmosphere refracts that single ray of light, the star appears to twinkle. If youre still not sure, its time to consult the sky chart for that night.
(Thanks to Brian Armstrong, of the National Solar Observatory/Sacramento Peak in Sunspot, New Mexico for his contribution).
Meteors - If a speeding object blazes across the sky and looks like its leaving a trail, its probably a meteor. Individual meteors can shoot across the sky any time. Meteors are usually small pieces of rock that skid through the atmosphere and burn upusually before they ever hit the ground.
Comets - Objects that appear to move over the course of several weeks and have a long tail and a fuzzy head are comets. Some comets, like Halleys Comet,
make routine reappearances after years or decades of being too far away to see.
Stars - Most of the objects we see at night are stars. They appear to move in concert across the sky, because the Earth is turning underneath them, but their real motions are difficult to detect because theyre so far away.
Galaxies and nebulae - Most galaxies and nebulae are too faint to see with the naked eye. The Andromeda galaxy and the Orion nebula are two exceptions. On a dark, clear night the Andromeda galaxy can be seen in the constellation of the same name. It is very far away, but it is very largeits diameter is nearly that of the moonand with a telescope or binoculars you can see it as a fuzzy patch of sky. The Orion nebula also looks like a fuzzy spot in the sky, but its shape is more irregular and in a telescope it appears to have more color.
You can get your bearings by finding a familiar star or constellation (be careful not to choose a planet for your starting place). Use the star or constellation to point to other objects. For example, locate the Big Dipper. The two stars at the edge of the cup opposite the handle will point toward the North Star, Polaris. You can also use the stars in the handle of the Big Dipper to point in the direction of the bright red star Arcturus. Keep following that path, and youll find Spica. Star hopping is one of the best ways to learn where things are in the night sky.
The brightness of starlight depends on several things: actual brightness, the stars distance from Earth, Earths atmosphere, to name a few. But astronomers have developed a brightness scale that can be helpful in locating stars.
Apparent magnitude - How bright a star appears when observed from Earth. The brightest stars have negative magnitudes, and the fainter ones have greater magnitudes. Heres a list of the three brightest stars in the Northern Hemisphere: