21stC Science Physics - P7

Observing the Sky

A sidereal day is the time taken for a star to return to the same position in the sky. This takes about 23 hours and 56 minutes.

A solar day is the time taken for the sun to appear at the same position in the sky. This is 24 hours. 

A sidereal day is 4 minutes shorter than a solar day.

Solar and sidereal days are different because the Earth orbits the sun as well as spinning on its axis.

The moon seems to go slower than the sun, taking about 24 hours to appear at the same position in the sky.

This is because the Moon orbits the Earth in the same direction as the Earth is rotating.

As the Earth moves around the sun, the direction we face changes slightly each day, therefore we can see a slightly different patch of the sky each night.

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 Moon doesn't glow itself, it only reflects the light from the sun.

As the moon orbits the Earth we see different amounts of the Moons dark and lit-up surfaces.

Lunar Eclipse

as the moon orbits the Earth it sometimes passes into the Earth's shadow. The Earth blocks sunlight from the moon, so almost no light is reflected from the moon and it just seems to disappear.

A total lunar eclipse is where no direct sunlight can reach the moon, otherwise there is a partial lunar eclipse or no eclipse at all.

Solar Eclipse

By chance, the moon is just the right size and distance away from the sun, that when it passes between the sun and the Earth, it can lock out the sun. This is a lunar eclipse.

From some parts of the Earth the sun is completely blocked out, this is a total solar eclipse. 

in other parts there is some light shining through, this is a partial solar eclipse.

however in most parts of the world the sun won't be blocked out at all.

Eclipses of any type do not happen very often.

This is because the moon orbits the Earth at a slight angle to the Earth's orbit around the sun.

For an eclipse to happen there needs to be a clear alignment, and because of the way that the moon and Earth are tilted, this does not happen very often.

Partial eclipses happen a bit more often as they don't need to line up perfectly.

Coverging Lenses

Converging lenses are fatter towards the middle and they cause rays of light to converge to a focus.

All lenses have a principle axis going through the middle of the lens.

The power of a lens can be calculated by:

-power= 1 / focal length 

Telescopes

Simple refracting telescopes use two converging lenses, an eye lens and an objective…