Glossary of Terms
Accuracy of Rise/Set Computations
Lengths of Day and Night
The Earth's Orbit around the Sun

HelpThe Earth's Orbit around the Sun

The Earth revolves around its own axis – an imaginary line that runs from the north pole to the south pole – once every 24 hours. Not only does the Earth revolve around its own axis, but it also orbits the Sun. The time the Earth takes to complete one journey around the Sun is one year, or to be more exact, 365.2422 days.

The Earth’s axis is tilted at around 23.45 degrees to the plane of its orbit around the Sun. It is this tilt that causes the seasons. The tilt changes in a cycle lasting 41,000 years. In 1900 the tilt was 23.45229 degrees; in August 1977 the tilt was 23.44229 degrees; in the year 2000 the tilt was 23.43928 degrees.

Due to the gravitational pull of the Sun and Moon, as the Earth spins on its axis, it wobbles like a spinning top (precession). A complete circuit takes around 25,780 years. The Moon also causes smaller wobbles having a whole range of periods from days to 18.6 years (nutation).

The Earth's orbit is a nearly circular ellipse in the same plane as the Sun. This plane is known as the Ecliptic Plane because when the Moon moves into the plane there is the possibility of an eclipse. One consequence of the elliptical orbit is that the Earth speeds up and slows down as it moves around the Sun, and this means that the length of the solar day, measured from noon to noon, changes throughout the year. Thus solar time differs from local time. The difference between solar time and local time is called the Equation of Time.

The Earth is nearest the Sun (perihelion) at around 3rd January and farthest (aphelion) at around 4th July. At its closest point, the Earth is 5,020,000 km or 3.3% closer to the Sun than when it is furthest away. One slight way the Earth's orbit changes is the movement of the Earth's perihelion. The Earth's perihelion moves in a complete circle about the Sun in 21,310 years. Because of this, the Earth's orbit can be visualized as a perpetual spiral, never quite visiting the same place twice in space.

The average distance of the Earth from the Sun is 149,600,000 km. The orbital speed averages 30 km/sec - is greatest at perihelion and is least at aphelion.