# Physics P7.1 Observing The Sky With The Naked Eye

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The Sun moves from East to West every 24 It is illuminated because it reflects light from
hours (a cool trick to remember this: "It is the Ayesha Khanom 11L the Sun.
East and Juliet is the Sun"), this is equal to a Physics The part of the Moon facing the sun is lit up;
SOLAR DAY. the part facing away is in darkness.
Like the Sun, stars also move from East to The phases of the Moon depend on its
West, but they do so in 23 hours and 56 position in relation to the Sun and Earth.
minutes. This is known as a SIDEREAL DAY. As it makes it way around the Earth, the
The planet Earth is at a 23.5° tilt towards the bright parts of the Moon's surface at
Sun, thus causing seasons on different P7.1 Observing the different angles, these are the `PHASES'.
hemispheres. sky with the naked It takes the Moon 29.5 days to completely
In accordance to the direction in which the cycle through all eight phases ­ New, Waning
Sun and stars move, the Moon also moves eye Crescent, 1st Quarter, Waxing Gibbous, Full,
from East to West. But, unlike the other two, Waning Gibbous, Last Quarter and Waxing
the Moon does this in 24 hours and 49 Crescent ­ known as a LUNAR Month.
minutes. It also rises four minutes later each The Moon orbits the Earth from West to East
night (if you saw the Moon at 23:57 today over the course of 28 days (it might appear
and wanted to see it in exactly the same to slip slowly backward through the pattern
position tomorrow, you'd see it at 0:50). of the stars).
The planets orbit the Sun. So their positions
A solar eclipse is caused when light from the appear to change night by night, relative to The position of stars and other astronomical
sun is blocked by the Moon. the background of stars. objects are measured in terms of angles as
When the Moon's full shadow falls on the For most of the time, the planets move in a seen from Earth.
Earth (the UMBRA) a total eclipse is seen. If steady pattern, but sometimes they seem to Two angles are needed to describe the
the shadow is only partial (PENUMBRA) we go backward. position of a star: Point at the horizon due
see a partial eclipse. This is caused by the fact that the Earth is North of your location; turn westwards
There's a rarity because Moon doesn't line also orbiting the Sun but at a different rate. through an angle until you are pointing at
up with the Sun frequently. Due to its orbit The direction that a planet is moving the horizon (this is the first angle); point arm
tilted by 5° relative to the plane of Earth's depends on where both the planet and the upwards through angle until pointing directly
orbit. Earth are in their orbits. at star (this is the second angle).
A lunar eclipse is when the Earth's shadow Scientists measure the angles to fractions of
blocks the sunlight from reaching the Moon. To an observer on Earth, it appears that Mars is
More common as Earth's shadow is larger following a looped path against the fixed stars. seconds of arc: a second of arc is of a