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What Is a Perihelion?

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  • Written By: Mary McMahon
  • Edited By: Shereen Skola
  • Last Modified Date: 01 November 2016
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A perihelion is the point in the Earth’s orbit where it is nearest to the Sun. The opposite is called aphelion, where the Earth moves furthest away. During perihelion, the Sun’s light tends to be more intense because of the increased proximity. This event typically occurs in January, several weeks after the winter solstice in the northern hemisphere. While it might seem like it should be summer for everyone when the Earth is close to the Sun, seasons are determined by the planet’s tilt, not its location in space.

The difference between perihelion and aphelion is not much, on the scale of the rest of space. At perihelion, the Earth is almost 92 million miles (about 147.5 million kilometers) from the Sun, while at aphelion, it moves close to 95 million miles (around 152.6 million kilometers) away. These relatively small distances are the result of the Earth’s nearly circular orbit around the Sun; in scientific terms, it is said to have “low eccentricity,” unlike planets like Pluto, which have much more exaggerated elliptical orbits.

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Dating can shift from year to year, as the Earth’s orbit around the Sun does not take exactly 365 days. Scientists can calculate the precise date and time of perihelion using predictive formulas and calendar adjustments like leap years. Periodically they may need to add leap seconds or other units of time to compensate for small variations that can add up over extended periods of time. They can also use this information to extrapolate backward to determine the date of perihelion at various points in history.

Researchers can use a variety of tools to measure distance at perihelion and to determine the intensity of the light on the Earth’s surface during this period. The Earth’s regular orbit is one of the reasons the plant is habitable, because it doesn’t go through extremes as it moves around the Sun. Seasons are a result of the planet’s off-center tilt, but remain comparatively mild. Planets with similar orbital characteristics might also potentially harbor life, and are sometimes targets for searches by astronomers.

Observers on Earth may not notice very much of a different during perihelion and aphelion. The Sun’s size would appear the same, for those who used tools to view it safely, like heavily filtered cameras. While the Sun’s light is more intense, the difference is not significant enough for people to notice on the ground without specialized tools.

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