Typed up notes taken from the CGP P7 Guide :)
P7 - Further Physics
Studying the Universe
Observing the Sky
A Sidereal Day is the Time Taken for the Earth to Spin Once
1. If you looked at the night sky for long enough, you’d see distant starts appear to cross the sky from east to west.
2. Astronomers have known for years that its not the stars that move, but the Earth that spins on its axis.
3. For a star to get to the same position in the sky, the Earth needs to spin 360 degrees. The time taken for this to happen is called a sidereal day.
A sidereal day is the time taken for a star to return to the same position in the sky. Its about 23 hours and 56 minutes.
The Sun and the Moon Appear to Cross the Sky at Different Speeds
1. Its not just the stars - the Sun and Moon also appear to cross the sky from east to west.
2. The sun seems to move more slowly across the sky than distant stars - it takes 24 hours to get to the same position in the sky, 4 minutes later. This is called a solar day.
A solar day is the time taken for the Sun to appear at the same position in the sky. It’s 24 hours.
3. Solar and sidereal days are different because the Earth orbits the Sun as well as spinning on its axis.
4. The Earth orbits the Sun in the same direction as it spins - so the Earth needs to spin slightly more than 360 degrees before the Sun appears at the same position in the sky.
5. The Moon seems to go more slowly than the Sun, taking about 25 hours to appear at the same position in the sky.
6. This is because the Moon orbits the Earth in the same direction as the Earth is rotating.
The Stars You Can See in the Sky Change During the Year
1. As the Earth moves around the Sun, the direction we face changes slightly each day.
2. This means we can see a slightly different patch of sky each night - we see different stars.
3. An Earth year is the time it takes the Earth to orbit the Sun once, so on the same day each year you should be able to see the same stars in the night sky.
Eclipses and the Moon
The Phases of the Moon
1. The Moon doesn’t glow itself - it only reflects light from the Sun. Only the half facing the Sun is lit up, leaving the other half in shadow.
2. As the Moon orbits the Earth, we see different amounts of the Moon’s dark and lit up surfaces.
3. You see a ‘full moon’ when the whole of the lit up surface…