Why does our view of the stars in the night sky change through the year? It changes because, as the Earth moves around the Sun, different parts of the Universe are seen at night. Thus, for example, in the northern hemisphere, the constellation Orion is prominent in the night sky of winters. Orion is not seen in summers because in summer the night-face of the Earth looks out in the opposite direction and Orion appears in the day sky instead.

A year is about 365.24 days long, not 365. It takes the Earth about 365.24 to complete a circuit around the Sun. Calendars, both ancient and modern, use intercalation, the insertion of an extra day (or days) during certain years, to keep themselves aligned with the seasons.

Because the axis of rotation of the earth is inclined to the plane of the Earth’s orbit around the Sun at 23.5 degrees, we have seasons. The plane in which the Earth orbits the Sun is called the ecliptic plane. At every latitude our distance from the Sun, and the thickness of the atmospheric shell that lies between us and the Sun, changes throughout the year. Because the Earth’s axis is inclined to the ecliptic plane at 23.5 degrees, we define the Tropic of Cancer to lie at 23.5 degrees north latitude, and the Tropic of Capricorn at 23.5 degrees south latitude. At the tropics the Sun is directly overhead in midsummer. These are the hottest places at noontime in summer. Intercalation keeps all three things aligned: our calendar, our seasons, and our view of the night sky.

The stars are themselves moving, relative to us, at very high speeds. Since they are so far away, they appear to us to move very slowly. In fact they were long thought of as being “fixed”. Still their motion adds up over the years. After 10,000 years, for example, the North Star (Polaris) will no longer remain the North Star.

The North Star, by the way, is a binary star system: two stars orbiting tightly around each other. Indeed a majority of the stars we see in the night sky are not single stars. The majority are binary systems. In our Astronomy comics we will explain how people find out if a star system consists of a single star or of two or more stars. We cannot see the individual stars that make up a binary system or a ternary system, they are all too far away for that, but we can tell nevertheless what sort of a system they are. It is in fact utterly fascinating how much we can find out about stars millions of light years away merely by looking at the photons they send us. We can tell how old they are. We can tell what they are made of. We can tell how far away they are. We can tell what their mass is. We can tell how large they are. So much, and more, from the few photons we harvest from their light. All this we will discuss and explain in our comics.