How The Sky Moves
Look up on a clear night and you’ll notice the sky looks like a vast hemispherical dome with stars fixed to its inner surface. If the Earth were transparent, you would see the stars on the other half of this starry dome, below your feet, and you’d get the impression you were standing at the center of a velvety-black sphere speckled with stars. Astronomers call this the celestial sphere.
As you know, the Earth spins in space, rotating once a day on its axis. But from an observer’s point of view, the Earth appears to remain still while the celestial sphere seems to rotate once a day about an axis that runs from the north celestial pole (NCP) to the south celestial pole (SCP), which are imaginary points above the Earth’s north and south poles. So all the stars, planets, moon, and sun on the celestial sphere also appear to move all the way around the sky once each day, rising in the east and setting in the west.
That North Star, Polaris, lies very near the rotation axis of the celestial sphere, right about the Earth’s north pole. Since it’s almost right on the north celestial pole, Polaris appears to stay fixed nearly fixed in the sky all night, and all year. Any other star on the celestial sphere south of Polaris rotates in circles of increasing diameter about the rotation axis.
Stars above the Earth’s equator trace out the circles with the largest diameter during their daily motion across the celestial sphere. And south of the equator, stars trace out circles with smaller apparent diameters as they lie closer to the south celestial pole. By chance, there is no bright star… no “Southern Star” that corresponds to Polaris… at the south celestial pole. This picture will give you a better idea of how the celestial sphere appears to rotate.
Like the stars and planets, the Sun also appears to move on the celestial sphere. If you measure the time when the sun is highest in the sky, you will find it takes exactly 24 hours for the sun to move all the way around the celestial sphere and return to its highest point. In fact, that’s how we define a “day”, or what astronomers formally call a solar day.
It’s a little different with stars. If you go out at night and select a star to observe, and measured its position on the celestial sphere, you will find it takes 24 hours to move all the way around the sky and get back to the same spot.
Well, almost 24 hours.
You see, if you measure accurately, you’ll find it takes only 23 hours and 56 minutes for a star to get back to the same position in the sky as it was the night before. That’s because, during the day, the Earth revolved around the sun by 1/365 of its orbit. So each day, you look in a slightly different direction in space such that every star appears to rise 4 minutes earlier each night. In two weeks, the star rises about an hour earlier; in one month the star rises 2 hours earlier, and in 12 months, it appears to move all the way around the sky back to the position at which you first saw it the previous year.
This apparent motion, where the stars rise a little earlier each night, explains why the stars you see in the winter sky are different than the stars you see in the spring, summer, and autumn sky.
Maps of the Earth are marked with latitude and longitude, a north-south east-west grid that helps us locate places on the Earth’s surface. Latitude measures in degrees how far north or south of the equator a place lies. By convention, the equator has a latitude of zero degrees, the north and south poles have a latitude of 90° north and 90° south, respectively. Chicago has a latitude of 41.8° north; Sydney, Australia has a latitude of about 34° south.
Longitude measures how far east and west a place lies on the Earth’s surface. But how far east and west of what? By convention, the reference point of longitude is the great circle that runs through the earth’s poles and the Royal Greenwich Observatory in London, U.K. So Greenwich is at zero degrees longitude. Chicago, west of Greenwich, has a longitude of 88° west. Sydney, east of London, is at a longitude of 151° east.
Why are we telling you this?
Because astronomers locate things on the sky using a celestial equivalent to latitude and longitude. Imagine the Earth’s equator and lines of latitude and longitude projecting upwards onto the celestial sphere. The celestial equator lies directly above the Earth’s equator, and the north and south celestial poles are above the Earth’s poles.
And lines and latitude and longitude are there as well. But in the sky, latitude is called declination. The celestial equator has a declination of 0 degrees. North and south of the celestial equator, declination is marked with a “plus” and “minus” sign. The star Vega, for example, has a declination of +38.8°. The southern star Achernar has a declination of about -57°.
Each degree is split into 60 smaller units called “minutes of arc”, marked by a ‘, and each minute is split into 60 “seconds of arc”, marked by a “. So the more precise declination of Achernar is -57° 14′ 12″.
The celestial equivalent to longitude is called right ascension. It’s measured not in degrees but in “hours”, from 0h to 24h. Astronomers cooked up this arrangement long ago because the celestial sphere appears to turn once every 24 hours. With 24 hours in the full 360 degrees of sky, each hour corresponds to 15 degrees of angular distance. Like degrees, each hour is split into 60 minutes, and each minute into 60 seconds.
The right ascension of Achernar, for example, is 01h 37m 43s; Vega is at right ascension 18h 36m 56s.
By convention, the great circle with right ascension of 0 hours runs through the a point in the constellation Pisces at which the ecliptic (see below) crosses the celestial equator, and right ascension increases going eastward.
So that’s the basics of celestial coordinates. You’ll see these numbers in astronomy books and magazines to describe the positions of objects on the celestial sphere. And you’ll see the coordinates marked on the pages of star atlases to help you find things.
So now you know the north and south celestial poles, and the celestial equator. There’s one more imaginary circle on the celestial sphere you need to know and which you’ll see on some star maps. It’s called the ecliptic, and it marks the approximate plane of the solar system in space in which the planets and sun are found. The Earth’s axis is tilted about 23.5° from this plane, and that’s the angle between the ecliptic and the celestial equator.
The ecliptic marks the path the Sun, Moon, and planets appear to follow across the sky during the year. It passes through 12 constellations collectively called the zodiac: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, Capricorn, Aquarius, and Pisces. (Actually, the ecliptic passes through a 13th constellation, Ophiuchus, but ancient astrologers considered the number 13 unlucky).
The ecliptic crosses the celestial equator at two points called the equinoxes. At the vernal equinox, the Sun appears to cross the equator moving northward in the sky. This marks the beginning of spring/fall in the northern/southern hemisphere. The vernal equinox is located in the constellation Pisces. At the autumnal equinox, the sun crosses the celestial equator going south, marking the beginning of fall/spring in the northern/southern hemispheres.
The local meridian is an imaginary great circle on the celestial sphere that’s perpendicular to the local Horizon. It passes through the north point on the Horizon, through the celestial Pole, up through the point directly overhead (the zenith), and through the south point on the horizon. A star or other object on the celestial sphere “culminates” when it crosses the meridian at the highest point above the horizon. That’s usually the best time to observe an object because it’s at the highest point above the horizon, which means you have to look through less of the dust and moisture of the Earth’s atmosphere.
The right ascension of a star crossing the meridian sets what’s called the local sidereal time. But that’s more advanced, and perhaps more than you need to know right now…