# The Story of Earth's Orbit Around the Sun

Earth's motion around the Sun was a mystery for many centuries as very early sky watchers attempted to understand what was actually moving: the Sun across the sky or Earth around the Sun. The Sun-centered solar system idea was deduced thousands of years ago by the Greek philosopher Aristarchus of Samos. It wasn't proved until Polish astronomer Nicolaus Copernicus proposed his Sun-centered theories in the 1500s, and showed how planets could orbit the Sun.

Earth orbits the Sun in a slightly flattened circle called an "ellipse." In geometry, the ellipse is a curve that loops around two points called "foci." The distance from the center to the longest ends of the ellipse is called the "semi-major axis," while the distance to the flattened "sides" of the ellipse is called the "semi-minor axis." The Sun is at one focus of each planet's ellipse, which means that the distance between the Sun and each planet varies throughout the year.

## Earth's Orbital Characteristics

When Earth is closest to the Sun in its orbit, it is at "perihelion." That distance is 147,166,462 kilometers, and Earth gets there each January 3. Then, on July 4 of each year, Earth is as far from the Sun as it ever gets, at a distance of 152,171,522 kilometers. That point is called "aphelion." Every world (including comets and asteroids) in the solar system that primarily orbits the Sun has a perihelion point and an aphelion.

Notice that for Earth, the closest point is during northern hemisphere winter, while the most distant point is northern hemisphere summer. Although there's a small increase in solar heating that our planet gets during its orbit, it doesn't necessarily correlate with the perihelion and aphelion. The reasons for the seasons are more due to our planet's orbital tilt throughout the year. In short, each part of the planet tilted toward the Sun during the yearly orbit will get heated more during that time. As it tilts away, the heating amount is less. That helps contribute to the change of seasons more than Earth's place in its orbit.

## Useful Aspects of Earth's Orbit for Astronomers

Earth's orbit around the Sun is a benchmark for distance. Astronomers take the average distance between Earth and the Sun (149,597,691 kilometers) and use it as a standard distance called the "astronomical unit" (or AU for short). They then use this as shorthand for larger distances in the solar system. For example, Mars is 1.524 astronomical units. That means it's just over one-and-a-half times the distance between Earth and the Sun. Jupiter is 5.2 AU, while Pluto is a whopping 39.,5 AU.

## The Moon's Orbit

The Moon's orbit is also elliptical. It moves around Earth once every 27 days, and due to tidal locking, always shows the same face to us here on Earth. The Moon doesn't actually orbit Earth; they actually orbit a common center of gravity called a barycenter. The complexity of the Earth-Moon orbit, and their orbit around the Sun results in the apparent changing shape of the Moon as seen from Earth. These changes, called phases of the Moon, go through a cycle every 30 days.

Interestingly, the Moon is slowly moving away from Earth. Eventually, it will be so far away that such events as total solar eclipses will no longer occur. The Moon will still occult the Sun, but it won't appear to block the entire Sun as it does now during a total solar eclipse.

## Other Planets' Orbits

The other worlds of the solar system that orbit the Sun have different length years due to their distances. Mercury, for example, has an orbit just 88 Earth-days long. Venus's is 225 Earth-days, while Mars's is 687 Earth days. Jupiter takes 11.86 Earth years to orbit the Sun, while Saturn, Uranus, Neptune, and Pluto take 28.45, 84, 164.8, and 248 years, respectively. These lengthy orbits reflect one of Johannes Kepler's laws of planetary orbits, which says that the period of time it takes to orbit the Sun is proportional to its distance (its semi-major axis). The other laws he devised describe the shape of the orbit and the time each planet takes to traverse each part of its path around the Sun.

Edited and expanded by Carolyn Collins Petersen.

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