Astronomy - Chapter 1 - Earth, Moon and Sun - SUN - EARTH - MOON INTERACTIONS

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  • Created on: 10-04-13 09:49
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  • Astronomy - Chapter 1 - Earth, Moon and Sun - SUN - EARTH - MOON INTERACTIONS
    • Lunar phase cycle
      • 29.5 days (one lunar month)
      • It starts with NEW MOON (when the Moon is aligned with the Sun)
      • Then waxes (through it's crescent and gibbous phases) until it is directly opposite the Sun and appears to be a full Moon
      • Then wanes (through gibbous and (de)crescent phases until the cycle ends with another new moon
      • Lunar Month
        • A lunar month is 2.2 days longer than the Moon's orbital period around the Earth
    • Eclipses
        • Moon moves infront of the Sun casting a shadow on the Earth
        • The Moon moves into the shadow of the Earth
        • A total eclipse is when a person is in the area of a Umbra
        • A partical Eclipse is when a person is in the area of a Penumbra
        • The Penumbra is the outer zone of partial shadow
      • The Sun diameter is 400 times large than the Moon and is 400 km further away than the Moon. So the angular sizes are almost the same
    • Sidereal Day and Solar Day
      • A sidereal day is 23 hours 56 minutes (and 4.1 seconds) and it is also the time taken for successive crossings of a given star across an observer's meridian
      • A solar day is exactly 24 hours. This is the time taken for successive crossings of the Sun across the observer's meridian
      • The extra 4 minutes brings Earth back in line with the Sun and starts a new day
    • Shadow Stick
      • Daily variations
        • Shadows move clockwise, indicating that the Earth is rotating in an anti-clockwise direction
      • Seasonal variations
        • Shadows are longer in the winter and shorter in the summer, showing that the angular height of the Sun varies during a year (the angle can be found to give a di"erence of 47?)
        • The shortest shadow is at noon on the summer solstice
        • The longest noon shadow is at the winter solstice
    • Sundial
      • A sundial uses light from the Sun
      • This means that it determines the APPARENT SOLAR TIME
      • Sundials read Local Solar Time
    • Seasonal Sun
      • The different angles of light from the Sun reaching a part of the Earth through a year gives rise to the seasons – not the distance the Earth is from the Sun (the Earth is nearest the Sun in early January due to its elliptical orbit).
      • In the summer in the northern hemisphere, light passes more directly into the atmosphere. In the winter, the light meets the atmosphere at a more oblique angle and some of the radiant heat energy is re?ected away from the planet
        • As a result, more energy enters the Earth in the northern hemisphere in summer than in winter, therefore making the summers hotter
      • Summer: The axis tilts towards the Sun and the Sun is at its highest point above the Tropic of Cancer on 21st June – the Sun’s heat energy passes directly into the atmosphere. The heat energy landing on the surface of the Earth is concentrated in a smaller area.
      • Winter: The axis tilts away from the Sun and the Sun is at its lowest point above the Tropic of Capricorn on 21stDecember - the Sun’s heat energy hits the atmosphere at an angle and some heat energy is re?ected away. The heat energy landing on the surface of the Earth in the northern hemisphere is spread over a wider area
    • Equation of time
      • Apparent solar time: Is the time that is shown on sundials. The Sun being off-centre in an elliptical orbit is what we actually see (this Sun is called the ‘apparent Sun’)
        • Apparent solar days are measured from noon one day to noon the next day
      • Mean Solar time: Instead of our Sun being o!-centre in an elliptical orbit, imagine a Sun at the centre of a circular orbit (this Sun is called a ‘mean Sun’ and the regular time resulting from a circular orbit is called ‘Mean Solar Time’)
        • Our time uses the mean solar time
      • Equation of time = apparent sun - mean sun
      • The Sun does not travel at a regular speed across the sky for two reasons
        • The Earth orbits the Sun in an elliptical path. When we are closer to the Sun in winter, the Earth is travelling faster
        • The Earth’s axis is tilted to the plane of the orbit. The Sun moves along the ecliptic and not the celestial equator - the Sun has to reach different heights above the horizon at noon each day
      • An apparent solar day may differ from a mean solar day (of 86,400s) by as much as nearly 22s shorter to nearly 29s longer
      • As many of these short or long days occur in succession, the difference builds up to as much as nearly 17 minutes ahead or a little over 14 minutes late


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