12 Celestial Sphere – Introduction

Astronomers map the sky. Maps of the sky have aided navigators for thousands of years. They also let astronomers locate and identify astronomical objects such as stars, nebulae, and galaxies and plan astronomical observations. Backyard astronomers use sky atlases (likely digital) to find interesting objects in the sky. Professional astronomers map the sky so that they can tell telescopes where to point, to create catalogs of stars and galaxies, and to combine multiple images of they sky into large mosaics, among other things.

If you watch the stars move over the course of the night, they look like they are all painted on a giant sphere that is slowly turning around the Earth. This happens because the Earth is rotating; spinning like a top around a line connecting the Earth’s North and South Poles. Astronomers call this imagined sphere the Celestial Sphere.

Ancient astronomers believed that the Celestial Sphere was real; that all of the stars rode on a perfect sphere that marked the edge of the Universe. We know now that the stars are spread out around us, stretching thousands of light years in every direction, and that stars and galaxies that are invisible to the naked eye fill a Universe that is at least billions of light years deep in every direction.

Modern astronomers kept the idea of the Celestial Sphere because it is perfect for creating a map of the sky. In someways, the Celestial Sphere is similar to a globe; it has North and South Celestial Poles and a Celestial Equator. The Earth has longitude and latitude; the Celestial Sphere has azimuth and declination. But there is one important difference. We do not walk along the surface of the Celestial Sphere; it isn’t even really there to imagine walking on. Instead, we look up at it from the Earth’s surface.

If someone standing on the Earth’s North Pole looks straight up, the direction that they will be looking is the North Celestial Pole. If you look in the same direction, you’ll also be looking at the North Celestial Pole, but you won’t be looking straight up. Instead, you’ll be looking at some angle above (or below) the northern horizon. When the sky is viewed from the Earth’s northern hemisphere, the North Celestial Pole is the point around which the stars appear to turn. (In the southern hemisphere, the stars appear to turn around the South Celestial Pole.) In the northern hemisphere, we luckily have a bright star that is almost directly on the North Celestial Pole. This star is Polaris, or the north star.

Polaris was extremely useful to early navigators, and it’s easy and fun to find in the night sky. Start by looking up your latitude; for example, Mesa, Arizona is located at about 33.4 degrees north. Next, go outside on a clear night. Face due north, then look up the same number of degrees above the horizon as your latitude. When your facing the horizon, that’s zero degrees. Craning your neck to look straight up is 90 degrees; half way in between is 45 degrees, and 33.4 degrees is just enough of an upward tilt to be slightly unpleasant. Now look for the little dipper, otherwise known as Ursa Minor. Polaris is the tail of the dipper.

As long as you don’t travel, Polaris will always be in this same spot on the night sky, no matter the season or the time of night. The Celestial Poles are the only points on Celestial Sphere that don’t appear to move. The rest of the sky changes both throughout the night and during the year. The nightly turning of they sky is caused by the Earth’s rotation; annual changes in the sky are caused by the Earth’s orbit around the Sun.

If you do travel, Polaris can help you determine where you are. Start by scanning the sky to find the little dipper (Ursa Minor), then find Polaris. Congratulations, you have determined which direction due north is. Now pull out your handy sextant and measure the angle between the horizon and Polaris; you’ve now determined your latitude. Want to know your longitude as well? Unfortunately, that’s another story.