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Earth’s Orbit Around The Sun

Diagram of the Earths orbit around the Sun. Credit: NASA/H. Zell

Diagram of the Earths orbit around the Sun. Credit: NASA/H. Zell

Ever since Nicolaus Copernicus definitely demonstrated in the 16th century that the Earth revolved around in the Sun, scientists have worked tirelessly to understand exactly what our relationship to it is. If in fact, this bright celestial body – upon which depends the seasons, the diurnal cycle, and all life on Earth – does not revolve around us, then what exactly is the nature of our revolutions around it?

For several centuries, astronomers have applied the scientific method to answer this question, and have determined that the Earth’s orbit around the Sun has many interesting characteristics.

First of all, the speed of the Earth’s orbit is 108,000 km/h, which means that our planet travels 940 million km during a single orbit around the Sun. The Earth completes one orbit every 365.242199 mean solar days, a fact which goes a long way towards explaining why need an extra calendar day ever four years (aka. a leap year).

The planet’s distance from the Sun also varies as it orbits. In fact, the Earth is never the same distance from the Sun from day to day. When the Earth is closest to the Sun, it is said to be at perihelion. This occurs around January 3rd, when the Earth is at a distance of 147,098,074 km. When it is at its farthest distance from the Sun, Earth is said to be at aphelion – which happens around July 4th where the Earth reaches a distance of 152,097,701 km.

Next, there is the nature of the Earth’s orbit. Rather than being a perfect circle, the Earth moves around the Sun in an extended circular or oval pattern. This is what is known as an “elliptical” orbit. The concept that was first observed by Johannes Kepler, a German mathematician and astronomer, and described in his seminal work Astronomia nova (New Astronomy).

After measuring the orbits of the Earth and Mars, he noticed that at times, the orbits of both planets appeared to be speeding up slowing down. This coincided directly with the planets’ aphelion and perihelion, meaning that the planet’s distance from the sun bore a direct relationship to the speed of their orbit. It also meant that both Earth and Mars did not orbit the Sun in perfectly circular patterns.

In describing the nature of elliptical orbits, scientists use a factor known as “eccentricity”, which is expressed in terms of a number between zero and one. If a planet’s eccentricity is close to zero, then the ellipse is nearly a circle. If it is close to one, the ellipse is long and slender.

Earth’s orbit has an of less than 0.02, which means that it is very close to being circular. That is why the difference between the Earth’s distance from the Sun at perihelion and aphelion is very little – less than 5 million km.

By one astronomical convention, the four seasons are determined by flanges, the solstices—the point in the orbit of maximum axial tilt toward or away from the Sun—and the equinoxes, when the direction of the tilt and the direction to the Sun are perpendicular. In the northern hemisphere winter solstice occurs on about December 21, summer solstice is near June 21, spring equinox is around March 20 and autumnal equinox is about September 23. The axial tilt in the southern hemisphere is exactly the opposite of the direction in the northern hemisphere. Thus the seasonal effects in the south are reversed.

In modern times, Earth’s perihelion occurs around January 3, and the aphelion around July 4 (for other eras, see precession and Milankovitch cycles). The changing Earth-Sun distance results in an increase of about 6.9% in solar energy reaching the Earth at perihelion relative 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. However, 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.

The Hill sphere (gravitational sphere of influence) of the Earth is about 1.5 Gm (or 1,500,000 kilometers) in radius. This is the maximum distance at which the Earth’s gravitational influence is stronger than the more distant Sun and planets. Objects orbiting the Earth must be within this radius, otherwise they can become unbound by the gravitational perturbation of the Sun.

 

Another interesting fact is the relationship between the Earth’s seasons and its orbit around the Sun. Basically, seasons on our planet are caused by a combination of two factors: the first being the Earth’s axial tilt; and the second having to do with its distance from the Sun during the orbital period.

Our planet’s vertical axis is tilted at an angle of 23.4° perpendicular to the Earth’s orbital plane. This means that one hemisphere is tilted towards the Sun at different times of the year, making one hemisphere slightly closer and the other farther away.

This means that when the Earth is at a certain place in its orbit, the northern hemisphere will be tilted toward the Sun and experience summer while the southern hemisphere experiences winter. Six months later, when the Earth is on the opposite side of the Sun, the northern hemisphere is tilted away from the Sun and the order is reversed.

 

Here are a couple of links that give you some more information about Earth’s orbit: one is about the ellipse and the other goes to NASA. Here on Universe Today we have a great article that gives you 10 interesting facts about Earth. Astronomy Cast offers a good episode about black holes unbalancing the Earth.

References:
NASA: Speed of the Earth’s Rotation
NASA Eclipse

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