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Our place in space

Timekeeping by the Earth, Moon and Sun

A clock

The rotation of the Earth, the revolution of the Earth around the Sun and the Moon around the Earth are the basis of terrestrial timekeeping. 

 

Illustration of the solar day



The solar and sidereal day

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



The sidereal day

The sidereal day is measured with respect to the stars. It lasts 23 hours 56 minutes and 4 seconds. This is the time between two successive passages of a star across the meridian - the line connecting the due north and south points on the horizon with the overhead point (the zenith).

The sidereal day compared to the mean solar day

 

 

 

 

 

The sidereal day and how it is measured (left) and the sidereal day compared to the mean solar day (right). Click on the images to enlarge

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

Star positions, Monday 22.30
Star positions, Tuesday 22.26
Star positions, Wednesday 22.22
  The difference between the solar and sidereal days means that a given star will rise
  four minutes earlier each day. The diagrams illustrates how the constellation of Orion
  reaches the same position four minutes earlier on each successive day.


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.

The motion of the Earth around the Sun results in the Sun appearing to move against the background stars
The motion of the Earth around the Sun results in the Sun appearing to move against the background stars. Click to enlarge
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.
 

The seasons 

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.

The seasons
The fixed tilt of the Earth's axis is responsible for the seasonsClick to enlarge
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.

June solstice
June solstice
Equinoxes
Equinoxesclick to enlarge
December solstice
December solstice
   Lengths of days and nights at different places on the Earth. See the key below for the    location of each place highlighted.

Tromso, Norway 69° 42'N
Greenwich 51° 30'N
Equator 0°
Sydney, Australia 33° 32' 23''S

In June
Tromso never rotates into the Earth's shadow, experiencing 24 hours of daylight
In Greenwich we experience long summer days and short nights
At the equator days and nights are of equal length
Sydney has short hours of daylight and long nights

Equinoxes
All points on the Earth experience days and nights of roughly 12 hours in length

In December
Tromso never rotates into daylight, experiencing continuous night
Greenwich has short winter days and long nights
At the equator days and nights are of equal length
Sydney has long hours of daylight and short nights


The Sun's energy over a year
The Sun's energy over a year

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.

Path of the Sun at mid-northern latitudes
Path of the Sun at mid-northern latitudes

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

1. Explain with a diagram why some places of the Earth have 24 hours of daylight and 24 hours of darkness for part of the year.


2. Rewrite and correct the following statement:

During the summer, the northern hemisphere of the Earth is tilted towards the Sun. This means that the UK is closer to the Sun and so the weather is warmer. In winter, it is tilted away from the Sun so the UK is further away and the weather is colder.


3. Mars has an orbit where its distance from the Sun ranges from 210 to 250 million km. The southern winter takes place when it is furthest from the Sun and southern summer occurs when it is nearest. What effect will this have on the seasons in each hemisphere?


4. Why do you think the view of the stars from Earth changes with the seasons?


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