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Our place in space
Timekeeping by the Earth, Moon and Sun
The solar day is measured using the passage of the mean Sun across the sky. It lasts 24 hours - the average interval between two successive midnights.
Looking down to the north pole, the Earth rotates in an anticlockwise direction
Each solar day the Earth rotates 360º with respect to the Sun. Similarly the Earth rotates 360º with respect to the background stars in a sidereal day. During each solar day, the motion of the Earth around the Sun means the Earth rotates 361º with respect to the background stars.
Daily star positions
The year and the calendar
The sidereal year is the time taken for the Earth to travel once around the Sun and return to the same place with respect to the background stars. It lasts 365.256 days.
Our calendar is based on the cycle of the seasons and the so-called 'tropical' year, which measures the time take for the Earth to travel from equinox to equinox. It lasts 365.242 days, around 20 minutes less than the sidereal year.The calendar
Almost the whole world now uses the Gregorian calendar. Most calendar years are rounded down to a length of 365 days, leaving an error of about ¼ day per year. After 4 years an extra correction day is added to make a leap year. This ensures that the calendar stays in step with the seasons.
Even this correction produces a small error of 0.003 days, which is corrected by a special rule for century years (1900, 2000 etc). These years are only leap years if they are exactly divisible by 400 with no remainder.
Earth's equator is tilted at 23.5º to the plane of its orbit around the Sun, the ecliptic. The axis of rotation of the Earth always points to the same direction, towards the north celestial pole.
Starting in December, the northern hemisphere of the Earth is tilted away from the Sun. North of the tropics, the Sun will appear to be lower in the sky and the days will be shorter, reaching a minimum length on December 21, the winter solstice. Sunlight hits the ground at a shallow angle, so the heat is spread out over a large area, making the weather colder. In the southern hemisphere, the Sun is high in the sky and it is summer.
By March, both hemispheres of the Earth have days and nights of similar length. In the north, the Sun will now be higher in the sky. At the vernal equinox on about 22 March, the Sun is above the horizon for around 12 hours over most of the Earth's surface. The northern spring and southern autumn begin this month.
North of the tropics, the northern hemisphere has the longest days during June, when it is tilted towards the Sun. The Sun is high in the sky, so its heat strikes the ground at a steep angle leading to warmer weather. The Sun reaches its highest point on 21 June, the summer solstice. At this time of year, the part of the Earth to the south of the tropics is entering winter.
In September, both hemispheres again have days and nights of similar length. At the autumnal equinox on 23 September, the Sun is again above the horizon for 12 hours across the globe. This month sees the onset of autumn in the northern hemisphere and spring south of the equator.
During the winter the Sun's radiation strikes the ground at a shallow angle and days are short. This results in cooler weather. However, during the summer the reverse is true; the Sun's radiation strikes the ground at a steeper angle, and the days are longer, resulting in warmer weather.
In winter the Sun rises in the south-east and sets in the south-west. At the equinoxes the Sun rises in the east and sets in the west. In summer the Sun rises in the north-east and sets in the north-west.
Questions to think about
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