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Physics LibreTexts

2.1: The Night Sky

The night sky is the inspiration for astronomy. Even though modern astronomy is carried out using large telescopes and electronic cameras, the beauty of the night sky is the reason most astronomers became hooked on their subject. Everyone can share this experience. Go far from any city or suburb, or travel into a wilderness area. You will see thousands of stars scattered like gems on a velvet backdrop. You will see the pale arch of the Milky Way. You will understand the awe that humans have felt for thousands of years when they contemplate the night sky.

The unaided eye is good enough to see nearly 6000 stars from a dark site, along with planets and the brightest nebulae. Take a good pair of binoculars and you will also be able to see many nebulae and star clusters. With a small telescope, you can resolve star clusters and see galaxies several million light years away. You can even try photography. Your daily newspaper may have a column on what is visible in the night sky. Also, you can get this information from the monthly issues of Astronomy magazine and Sky and Telescope magazine. No matter what you might read or learn about astronomy, there is no substitute for going out and looking at the sky!
 


A time exposure centered on Polaris from Powell Butte, Oregon. High flying jet crossed during exposure. Click here for original source URL.

The Earth rotates eastward, so the sky appears to rotate above us from east to west. Stars rise in the east and set in the west. All stars appear to rotate in concentric circles around a fixed point in the sky near the bright star Polaris. Star motions appear imperceptible from minute to minute, but over the course of an hour or so you can see new constellations rise in the east while others set in the west. Planets are bright objects that do not twinkle and change their position among the stars from night to night. You may even see transient events in the upper atmosphere like shooting stars or fireballs.

Even though we know that the stars are all at different distances, it is convenient to imagine them on a celestial sphere that rotates overhead. You can orient yourself by finding Polaris, which is in the direction of the north celestial pole. The north celestial pole is directly above the direction that defines north on your horizon. Now you can find the cardinal points — north, south, east and west. The celestial equator is the projection of the Earth's equator onto the sky. If you are in the northern hemisphere, you will see it in your southern sky. The point directly above your head is the zenith and the point directly below your feet is the nadir. You can easily measure approximate angles on the sky. The angle between the zenith and the horizon, or between any two adjacent cardinal points, is 90°. The width of your fist held at arm's length is about 10°. The width of your thumbnail at arm's length is about 1°, and the width of the full Moon is 1/2°.
 


The azimuth is the angle formed between a reference direction (North) and a line from the observer to a point of interest projected on the same plane as the reference direction orthogonal to the zenith. Click here for original source URL.

You can locate any object on the sky with two angles. This is a direct analogy to the coordinate system of the Earth, where any place is specified by the two angles of latitude and longitude. One simple coordinate system in astronomy uses altitude and azimuth. The altitude of an object is its angular distance above the horizon, measured from 0° at the horizon to 90° overhead. The azimuth of an object is its angular distance around the horizon, measured towards the east from the north, from 0° to 360°.

Astronomers most often locate objects in the sky with a pair of angles defined in the equatorial coordinate system. The equatorial system is tied to the Earth as a frame of reference. The angle of an object above or below the celestial equator is called the declination — measured in degrees, minutes of arc, and seconds of arc. The declination is positive north of the celestial equator and negative south of the celestial equator. For example, suppose that your latitude is +28° N. A star with a declination of +28° would pass directly overhead, a star with a declination of +65° would pass to the north of the zenith, and a star with a declination of -21° would arc through the sky to the south of the zenith. Stars with declinations of less than -62° (in other words 28°-90°) are never visible because they never rise above the horizon. Telescopes need to point very accurately to find faint objects. A full description of declination might be +47° 15' 58", or just over 47 1/4 degrees north of the celestial equator.

The east-west coordinate is more complicated because the Earth is rotating. Astronomers use right ascension — measured in hours, minutes, and seconds of time. The reference point for right ascension is the time when an object crosses the meridian, which is the north-south line that passes overhead. The zero for right ascension is defined as the position of the Sun on the vernal equinox, which is around March 21 each year. Right ascension increases toward the east, because that is the direction in which the Earth turns. The range is 0 to 24 hours, and a particular right ascension might be expressed as 6h 47m 56.4s.

For an object on the celestial equator we can relate right ascension to angle as follows:

• 24 hours full rotation of the night sky = 360°

• 12 hours time between rising and setting of a star = 180°

• 1 hour typical angle between constellations = 15°

• 4 minutes time for star to cross 2 Moon diameters = 1°

• 1 minute time for star to cross 1/2 Moon diameter = 1/4°

telescope has to "track" an object; in other words, to remain pointing at a fixed right ascension, it has motors to move it in compensation for the Earth's motion. The orbital motion of the Earth makes the Sun appear to move eastwards among the stars. The path of the Sun in the sky is called the ecliptic. Each day the Sun moves about 1° (twice its angular diameter) eastward along the ecliptic. Each night at a particular time, the constellations are seen about 1° farther to the west. In other words, stars rise or set 4 minutes earlier each night. On March 21, the Sun is at a right ascension of 0h and stars at a right ascension of 12h are overhead at midnight. On September 21, the Sun is at a right ascension of 12h and stars at a right ascension of 0h are overhead at midnight. In this way, you can see different sets of constellations as the Earth sweeps around in its orbit of the Sun.
 


Revolving star chart (Planisphere) published by George Philip & Son, Ltd., London, around 1900. Click here for original source URL.

Most star charts show the celestial equator with right ascension marked off in hours. The ecliptic is the path followed by the Sun so it defines the plane of the Earth-Sun orbit. Since the solar system is nearly in a plane, all the planets can be found within a few degrees of the ecliptic. Mercury, Venus, Mars, Jupiter and Saturn are visible by eye. Uranus and Neptune are visible through binoculars. You will need a small telescope (and a dark site) to see Pluto. The celestial equator and the ecliptic intersect with an angle of 23.5° between them, because that is the tilt of the Earth's axis as it orbits the Sun. The planets move among the stars from night to night. Mercury and Venus are interior to the Earth's orbit so they never appear far from the Sun. The outer planets may be found anywhere along the ecliptic.

Viewers at northern latitudes see a certain set of constellations; additional constellations are only visible to viewers in the southern hemisphere. When you look at the sky you will have to use your imagination! The constellations along the ecliptic include the twelve signs of the zodiac. These have always been the most famous of the 88 constellations in the modern sky.