What do you see?

The night sky is a vast expanse, but what do you actually see when you look up at night?

From: www.telegraph.co.uk

From Earth, we can see more than 2000 different stars in the Milky Way galaxy. We also see constellations in the sky. Constellations are regions of the sky. The patterns of stars, for example the Big Dipper, are called asterisms. 

From: pics-about-space.com

There are 88 recognized constellations in the sky. The stars in each constellation are often varying distances from the Earth. The constellations appear as projections, and the stars in each region only appear close together as a result of these projections. 

The ancient Greeks believed that the stars and planets were all on a celestial sphere surrounding Earth. Although this is incorrect, the illusion is useful as it allows us to visualize the sky as seen from Earth. There are four important reference points on the sphere: the north celestial pole, the south celestial pole, the celestial equator, and the ecliptic. The north and south celestial poles are the points directly over the Earth's north and south poles. The celestial equator is a projection of the Earth's equator into space and it makes a complete circle around the celestial sphere. The ecliptic is the path that the Sun follows as it appears to circle the celestial sphere once each year. The apparent movement of the Sun is due to the Earth's orbit around the Sun. It can also be thought of as the path that the Sun takes through the constellations each year. The ecliptic crosses the celestial equator at the same angle as the tilt of the Earth't axis, which is 23.5 degrees.

From: www.daviddarling.info

Polaris, a.k.a the North Star, can be found roughly over the north celestial pole and the Southern Cross can be found roughly over the south celestial pole. If you are watching the sky for an extended period of time, the stars appear to be slowly rotating. They appear to make a complete circle around the north and south celestial poles, depending on where you are on Earth. Stars appear to rotate from east to west. For example, stars appear to rotate around Polaris in the northern hemisphere, or the point almost directly over the north celestial pole. However, the stars and Sun are actually stationary. The Earth is actually rotating and spins on its axis once every 24 hours. 

The sky that is visible to you depends on your location, specifically your latitude (north and south coordinate). Objects in your local sky can be found using its altitude (angle above the horizon) and its direction or azimuth (along the horizon). Some stars are visible all the time, some stars are never visible, and some stars are visible only part of the time. This is due to the tilt of the Earth's axis and the rotation of the Earth. For example, Polaris is always visible in the northern hemisphere because it is circumpolar (it never sets at certain latitudes). On the other hand, the Southern Cross is never visible in the northern hemisphere because it is circumpolar to the southern hemisphere, meaning it is always below the horizon in the northern hemisphere.

Depending on what your latitude is, you see different constellations. As the Sun appears to move through the sky, only certain parts of the sky are visible at certain parts of the year. For example, in the northern hemisphere, during the summer, we see Scorpius and during the winter, we see Orion. In fact, according to Greek mythology, Orion and Scorpius were put on opposite sides of the sky because a scorpion killed Orion and Orion is supposedly fleeing across the sky. In the southern hemisphere, the opposite is true (Orion is visible in the summer and Scorpius is visible in the winter).

It is important to comprehend the appearance of the night sky because it can provide valuable information about the motion of the earth and its orbit and rotation.