A sidereal day is the time take for the Earth to spin once.
- Stars appear to cross the sky from East to West.
- It's not the stars that move, but the Earth spins on its axis.
- For a star to get to the same position in the sky, the Earth must spin 360°. The time taken for this to happen is a sidereal day.
- A sidereal day is about 23 hours and 56 minutes.
Sun and moon
- The sun and moon also appear to cross the sky from East to West.
- 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, a whole 4 minutes longer. This is 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.
- Solar and sidereal days are different because the Earth orbits the the sun as well as spinning on its axis.
- The Earth orbits the sun in the same direction as it spins- so the Earth needs to spin slightly more than 360 before the sun appears at the same position in the sky.
The moon seems to go more slowly than the sun, taking about 25 hours to appear at the same position in the sky.
This is because the moon orbits the Earth in the same direction the Earth is rotating.
The stars you can see in the sky can change during the year.
- As the Earth orbits the sun the direction we face changes slightly each day.
- This mean each night we are in a slightly different patch of sky, so we see different stars.
- An Earth year is the time it takes for 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.
Phases of the moon
- 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.
- As the moon orbits the Earth, we see different amounts of the moons dark and lit-up surfaces.
- You see a full moon when the whole lit-up surface is facing the Earth and a new moon is when the dark half faces us.
- The other phases are in between these two phases.
Eclipses happen when light from the sun is blocked. There are two types:
- The moon passes into the Earths shadow.
- The Earth blocks sunlight from reaching the moon, so almost no light is reflected, the moon appears to disappear.
- A total lunar eclipse is where no direct sunlight reaches the moon.
- More often the moon isnt fully in the Earths shadow, so only part of it appears dark- a partial lunar eclipse.
- Moon passes between the sun and the Earth.
- It can block out the sun.
- From most parts of the Earth the sun won't be blocked out at all.
Eclipses are rare
- Eclipses aren't very common.
- The moon orbits the earth at an angle to Earths orbit around the sun. So most of the time the sun, moon and Earth don't line up.
- Partial eclipses happen more often as they don't have to line up perfectly for this.
- Even when there is a solar eclipse there is only a small region on Earth from which it can be seen.
Measuring positions of stars
- The positions of stars are measured by angles seen from Earth.
- The sky appears to turn as the Earth spins- astronomers picked two fixed positions to measure from:
- The pole star is a star that doen't seem to move because it's almost directly above the North pole (and the axis) of the Earth.
- The celestial equator is an imaginary plane running across the sky, extending out of the Earths equator.
Right ascension and declination
- The two angles used to measure angles in the sky are:
- Declination- celestial latitude, measured in degrees.
- Right ascension- Celestial longitude, measure in degrees or time.
- All planets in the solar system orbit the sun in the same direction but at different speeds. The closer to the sun the quicker the planet.
- Without a telescope we can often see the 'naked eye' planets- Mercury, Venus, Mars, Jupiter and Saturn.
- The planets seem to gradually travel from West to East.
- Every so often a planet seems to change direction and go the opposite way for a while (in relation to fixed stars), making a loop in its track before carrying on as normal. This is called retrograde motion. It only happens with the outer planets, Mars and Neptune.
- It happens because both the planet and Earth are moving around the sun- so we are seeing the motion of the planet relative to the Earth.
- Mars appears to change direction every two or so years. Slower moving planets, further away seem to 'change direction' less often.