# Physics P7

Physics P7

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## Observing The Sky

A sidereal day is the time taken for the earth to spin on its' axis ONCE.                 23 hours and 56 minutes

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

Solar and sidereal days are different because the earth orbits thr sun ASWELL at spinning on it's axis. The earth needs to spin slightly more than 360 degrees before the sun appears at the same positiion in the sky

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## Eclipses & The Moon

The moon reflects light from the sun.

You see a 'full moon' when the whole of the lit up surface of the moon is facing the earthand the 'new moon' when the dark half faces us.

Eclipses happen when light from the sun is blocked.
There are two types - LUNAR & SOLAR

LUNAR: as the moon passes into the earth's shadow the earth blocks sunlight to the moon, so almost no light is reflected from the moon and it just seems to disappear.

SOLAR: when the moon passes between the sun and the earth it can block out the sun.

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## Coordinates In Astronomy

The positions of stars are measured by angles seen from earth.

Asonomers picked two fixed positions to meaure from:
THE POLE STAR & THE CELESTIAL EQUATOR

The pole star does not seem to move as it's almost directly above the North Pole (and spin axis) of earth

The celestial equator is an imaginary plane running across thr sky, extending out formt he earth equator.

The two angles used to measure positions in the sky are are:
Declination - celestrial lattitude measured in degrees
Right Ascension - celestrial longitutde measured in degrees or time

All planets in the soloar system orbit the sun in the same direction, the closer to the sun, the quicker the planet moves

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## Converging Lenses

Converging lenses bring light rays together.

A converging lense is convex - it gets fatter towards the middle, this causes rays of light to converge (come together) to a focus

The line that passes straight through the middle of a lense is called a principle axis. All lenses have this.

Distance between the middle of the lense to the focal point = the focal length

Focal length is realted to power, thr shorter the focal length the more powerful the lense and the more strongly it converges rays of light.

Power = 1/Focal Length

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