Last lecture we saw what happens with the rotation of the Earth (day/night). Now lets do what happens as the Earth orbits around the Sun (year).
The most dramatic effect of course are the effect this has on the apparent motion of the sun, giving rise to the seasons. The motion of the Earth around the Sun during the year then makes the Sun appear to move against the background of stars. The orbit of the Earth then gets projected on the sky as the circular path of the Sun across the celestial sphere - called the ecliptic. The ecliptic is then at an angle of 23.5 degrees to the celestial equator. Constellations (join the dots) which are on this projected path are the constellations of the zodiac. More on the apparent motion of the sun.
In summer the pole of the earths rotation axis points towards the Sun so we see the sun at its maximum height above the celestial equator. The sun rises earlier, sets later and has a bigger height above the horizon when its on the meridian. The greater maximum height of the sun above the horizon means that the sunlight hits the earth at a rather small angle. In winter the pole of the earths rotation axis still points in the same direction, but we are now on the other side of the sun so it points away from the sun. The sun is at its maximum distance below the celestial equator, so it rises later, sets earlier and has a smaller height above the horizon when on the meridian. The sunlight makes a larger angle with the earth, so is spread over a larger area, so its cooler. Another reason its warmer in summer is that the daylight hours are longer, so the ground has longer to heat up. More on the seasons Equinoxes (equal nights) are where the sun crosses the celestial equator. Solstices (sun standing still) is where its at its maximum difference from the celestial equator.
But theres also a difference in the stars that can be seen. The Earth goes round in its orbit once a year, so at the end of a year all the stars are back in the same places. A circle is 360 degrees, while the Earths orbit is 365 days. So the Earth moves by about 1 degree per day (the angular size of the disk of the sun is 0.5 degrees). So the time between the sun making 2 sucessive crossings (called transits) of the meridian is a solar day of 24 hours (by definition). But for the stars, the time taken for them to make 2 sucessive transits of the meridian is NOT 24 hours. The Earth moves around the sun by 1 degree per day - 1 earth rotation is 360 degrees in 24 hours. so 1 degree is 24/360 hours = 0.06666 hours = 4 minutes. Thus the stars rise 4 minutes earlier each night - a sidereal day is 23 hours and 56 minutes long. That means a given star or constellation will rise 2 hours earlier at the end of the month than at the beginning (12 months, and they have to rise at the same time of day a year later). More on solar and sidereal time.
While the star patterns can be thought of as fixed, there are some very slow changes. The earth wobbles on its rotation axis like a spinning top - its called precession. It takes 26,000 years for it to do one complete wobble! So 13,000 years the bright star vega will be the pole star rather than Polaris.