The solar system is the collection of planets, comets and objects that orbit the sun.
Earth rotates once on axis: 23 hours 56 minutes
Earth orbits the sun: 365 1/4 days
A solar day: 24 hours
Moon moves across the sky once: about 25 hours
Moon orbits the Earth : About 28 days
Difference between solar and sidereal day: 4 minutes
Measuring stars positions.
Positions of stars are measured in terms of angles. Two angles are needed ( across and up).
An astrolabe is an ancient piece of equipment used for measuring angles. It was used for navigation.
Scientists measure these angles to fractions of seconds of arc. (1 degree = 3600 seconds of arc).
Most stars are so far away that there relative positions never change. These stars are called Fixed stars.
Movement of stars.
Earths rotations causes stars to appear to move.
A sidereal day is when the stars appear in the same place again after one rotation by the Earth. ( 23 hours 56 minutes)
A group of stars that form a pattern is called a: Constellation.
We see different constellations in summer and winter because of the earths orbit around the sun.
Movement of the sun.
The sun appears to move acorss the sky east to west.
It reappears in the same place every 24 hours. This is called a Solar day.
A solar day is 4 minutes longer than a sidereal day because: During one roation on the Earths axis it has also moved further along its orbit of the sun, so it has to rotate another 4 minutes untill the sun is in the same place again.
Movement of the moon.
The moon moves across the sky east to west. It takes 24 hours 49 minutes to reappear in the same place again.
The moons orbit of the Earth causes it to appear to slip back. Meaning it takes longer for it to reappear in the same place again.
A Solar eclipse is when sunlight coming to Earth is blocked by the moon.
Umbra: is when the moons full shadow falls on Earth. Penumbra is when the moons causes a partial shadow on the Earth.
Solar eclipses are rare because: The moons orbit is tilted 5' relative to the plane of the Earths orbit.
A Lunar eclipse is when the Earth blocks sunlight from the moon. It is more common than a solar eclipse because the Earth is much larger than the moon.
During a lunar eclipse the Moon appears red This is because sunlight is being refracted by the Earths atmospere so it can still reach the moons surface.