Lessons · The sky · 1

The Celestial Sphere: Horizon, Zenith, Directions

Goal: After this lesson you will be able to find your way around the sky using the horizon, the zenith and the four directions, and describe where an object is.

The sky as a giant bowl

On a clear night, the stars seem to be fixed on a huge dome around you. Astronomers call this imaginary globe the celestial sphere. It is not a real object: stars lie at very different distances from us. But as a model for finding your way, it is wonderfully handy. It has been used for centuries, and it is used in the Astronativa 3D sky too.

The horizon and the zenith

The horizon is the line where the sky seems to meet the land or the sea. It separates the part of the sky you can see from the part hidden below your feet.

The zenith is the point directly overhead, at the greatest height above the horizon: 90° from it in every direction. The opposite point, beneath your feet, is the nadir.

Every observer has their own zenith. If you are in Sofia and a friend is in Burgas, your zeniths point to slightly different places among the stars.

The four directions

The horizon has four main points: north, east, south and west. The Sun rises roughly in the east and sets roughly in the west (exactly east and west only around the equinoxes). The rest of the year the rising point shifts, which is linked to the seasons.

If you do not know where north is, in the Northern Hemisphere Polaris, the North Star can help. It sits close to the north celestial pole. You can find it with the help of the Big Dipper, a star pattern in the constellation Ursa Major (the Great Bear). Face it and north is ahead, south behind you, east on your right and west on your left.

Describing where something is

To say where an object is, astronomers use two numbers:

• Azimuth – the direction along the horizon, measured in degrees. North is usually 0°, east 90°, south 180°, west 270°.
• Altitude – the angle above the horizon: 0° at the horizon and 90° at the zenith.

For example, an object at azimuth 90° and altitude 30° is in the east, one third of the way up to the zenith. Your hand is a handy ruler: a fist at arm’s length is about 10°.

Why the sky turns

Stars rise and set because the Earth rotates on its axis, so the celestial sphere seems to turn around us. That is why the Sun and the Moon rise in the east and travel towards the west. How long an object stays above the horizon depends on your location and the time of year.

An observing tip

Choose a place with an open horizon, ideally away from strong lighting; there are more tips in How to watch the sky. Take your time: eyes need about 20 minutes to adapt to the dark, and then the sky reveals much more.

You now have the observer’s basic vocabulary: horizon, zenith, directions, azimuth and altitude. It will make every later idea easier.

Exercise in the sky

1. 1. Open the Astronativa 3D sky and set your location. Find the horizon and the directions north, east, south and west. 2. Turn the view to look straight up at the zenith. Describe what is there right now. 3. Pick a bright object and note its approximate azimuth and altitude. 4. Go outside after sunset, find north (for example with Polaris) and use your fist to measure the altitude of the same or another bright object. 5. Compare your observations with the 3D sky and notice how close they are.

Check yourself

Positions in Astronativa are astronomically exact. The interpretation is for inspiration. · Glossary · Terms · Privacy · Thank you