Earth Orbit

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The motion of the Earth around the Sun is called Earth orbit. The planet stands about 150 million kilometers from the Sun and completes an orbit every 365.242199 mean solar days. The extra time in each orbit causes the need for a ”leap year”. This motion causes the Sun to appear to move 1° across the sky each day. The Earth orbits the Sun at a speed of 108,000 km/h.

Because of the Earth’s axial tilt (obliquity of the ecliptic), the inclination of the Sun’s trajectory in the sky varies over the course of the year. For an observer at a northern latitude, when the northern pole is tilted toward the Sun the day lasts longer and the Sun climbs higher in the sky. This results in warmer average temperatures from the increase in solar radiation reaching the surface. When the northern pole is tilted away from the Sun, the reverse is true and the climate is generally cooler. Above the arctic circle there is no daylight at all for part of the year. This is called polar night. The axial tilt is the cause for the seasons.

The perihelion and aphelion of the Earth have changed over the course of history. In modern times, Earth’s perihelion(147,098,074 km) occurs around January 3, and the aphelion around July 4 (152,097,701 km) . The changing Earth-Sun distance results in an increase of about 6.9% in solar energy reaching the Earth at perihelion as related to aphelion. Since the southern hemisphere is tilted toward the Sun at about the same time that the Earth reaches the closest approach to the Sun, the southern hemisphere receives slightly more energy from the Sun than does the northern over the course of a year. This effect is much less significant than the total energy change due to the axial tilt, and most of the excess energy is absorbed by the higher proportion of water in the southern hemisphere.

For many centuries it was thought that the Earth did not orbit anything, but that everything orbited the Earth(geocentric universe). This view was only abandoned in the last 5 centuries. The heliocentric view(everything orbits the Sun) has taken over and many great scientific advances have been made since. Here is a link to the NASA webpage about Earth. We have a great article here on Universe Today about low Earth orbit. Astronomy Cast offers a good episode about solar system movements.

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