What to Look For

What you see in the night sky depends on your location, the season, and the time.

Planets orbit the Sun, so they change position when compared to the stars. The planet Venus, sometimes called the "evening star" or "morning star," is the second-brightest object in the night sky — only the Moon is brighter. Jupiter, Mars, and Saturn often are visible later at night than Venus. Planets reflect a steady light, unlike the "twinkling" stars, which are much farther from Earth.

The Moon is the most prominent object in the night sky. One half (or side) of the Moon is always lit by the Sun, but we can't always see the entire lighted part, which is what gives the Moon its phases. Most calendars show Moon phases, and many daily newspapers tell you when the Moon will rise and set.

How do I locate stars and planets in the night sky?

For the casual observer, locating stars, planets, and other astronomical objects in the night sky is most easily accomplished with the help of a star map. Many good books are available containing extensive maps of star positions for the entire sky, while magazines such as StarDate, Astronomy, and Sky & Telescope provide monthly maps that show the locations of the planets.

More careful observations require an understanding of the scales astronomers use to measure positions on the "surface" of the night sky. There are two primary coordinate systems used by astronomers; the first method employs a grid of right ascension and declination on the celestial sphere in the same manner that longitude and latitude are used on Earth's spherical surface.

A more intuitive system describes positions by their altitude in degrees above the local horizon and compass heading in degrees from 0 to 360, called azimuth. The flaw of this system is that the coordinates depend entirely on the location of the observer, and furthermore each object's position changes throughout the night as Earth turns in towards the dawn. The simplicity of this system, however, underlies its appeal and common usage; most "stargazing tips" and calendars refer observers to positions on their local horizon and how high above the horizon to look -- the altitude-azimuth system hard at work.

A helpful tool for using either of these two methods of finding your way around is found conveniently at the end of your arm. By extending your hand at arm's length, you can approximate a many degree measurements: The apparent width of one finger at arm's length, for instance, is about one degree, and a fist covers about 10 degrees.

So with a star map, a working knowledge of the coordinate systems, and maybe a few waves of the hand, any observer can find almost any object visible.

Why do planets change position in the night sky?

The planets appear to change position against the "fixed" stars in the night sky because of the relative motion of both Earth and the planets as we all move along in our orbits around the Sun. The swift-moving inner planets Venus and Mercury can move from morning to evening appearances and back again in as little as a few months. The giant outer planets such as Jupiter or Saturn, with orbits taking decades to complete, appear to move more slowly against the background of stars.

What is the difference between a morning star and an evening star?

What many people call the morning or evening star is actually a planet, usually Venus but sometimes Jupiter, Saturn, or Mars. When one of them appears close to the Sun as viewed from Earth, sunset or sunrise watchers are treated to starlike brilliance of one of our planetary neighbors, which may be brighter than any of the true stars in the night sky. The distinction between "morning" and "evening" simply refers to the time at which the planet is visible.

Due to the orbital motion of planets around the Sun, a planet might be a "morning star" at one time of the year, and then later, as its orbit carries it behind and then to the other side of the Sun, it appears as an "evening star."

Is the Milky Way visible from Earth?

Buried as we are deep within the spiral arms of our galaxy, our view is definitely more "trees" than "forest", but the starry girth of the Milky Way is one of the most spectacular and awe-inspiring sights in the night sky. The best time for viewing the Milky Way is mid- to late summer and again in midwinter, when the diffuse glow of hundreds of star clouds arch almost directly overhead in the early evening hours. Dark locations -- as usual -- are preferable, but the bright swath of the Milky Way is visible even from suburban areas.

Are any artificial satellites visible from Earth?

Under dark skies, you may be able to see one or even several of the hundreds of artificial satellites orbiting Earth almost any night of the year. Look for a faint starlike object moving steadily across the sky. Because the light from the satellite is entirely reflected sunlight, satellites will frequently "blink" in and out as they rotate and different areas of their surfaces face the Sun. Furthermore, as they pass into the darkness of Earth's shadow, they appear to vanish altogether. Spotting geostationary satellites, whose positions remain fixed above Earth, is a bit trickier and requires exact location data.

The International Space Station also is visible from Earth.

FacebookTwitterYouTube

©2014 The University of Texas McDonald Observatory