Happy Closest to the Sun Day – Perihelion 2021!

This composite neatly compares two pictures of the Sun taken with the same telescope and camera on the dates of Perihelion (closest approach) and Aphelion in 2008. The image labels include Earth's distance in kilometers from the Sun on the two dates. (credit: NASA APOD/Enrique Luque Cervigón)

January 2, 2021 at 8:50 a.m. EST (13:50 UTC), all of us on Earth and in orbit around Earth will be at our closest point to the Sun for the year. This point of closest approach to the Sun is called perihelion. The point when Earth is farthest from the Sun is called aphelion.

These terms are specific to a body orbiting the sun. For satellites of Earth (including the moon), the points of farthest and closest approach are called apogee and perigee, respectively. The generic terms for farthest and closest approach of a body in an elliptic orbit about a larger body are apoapsis and periapsis, respectively.

Aphelion versus perihelion. (orbits exaggerated). Image credit: NOAA/NASA.

Aphelion versus perihelion. (orbits exaggerated). Image credit: NOAA/NASA.

The diagram below shows the times and positions of the Earth during the solstices, equinoxes, perihelion and aphelion as it orbits the sun over a year.

This diagram shows the relation between the line of solstice and the line of apsides of Earth's elliptical orbit. The orbital ellipse (with eccentricity exaggerated for effect) goes through each of the six Earth images, which are sequentially the perihelion (periapsis—nearest point to the sun) on anywhere from 2 January to 5 January, the point of March equinox on 20 or 21 March, the point of June solstice on 20 or 21 June, the aphelion (apoapsis—farthest point from the sun) on anywhere from 4 July to 7 July, the September equinox on 22 or 23 September, and the December solstice on 21 or 22 December.

This diagram shows the relation between the line of solstice and the line of apsides of Earth’s elliptical orbit. The orbital ellipse (with eccentricity exaggerated for effect) goes through each of the six Earth images, which are sequentially the perihelion (periapsis—nearest point to the sun) on anywhere from 2 January to 5 January, the point of March equinox on 20 or 21 March, the point of June solstice on 20 or 21 June, the aphelion (apoapsis—farthest point from the sun) on anywhere from 4 July to 7 July, the September equinox on 22 or 23 September, and the December solstice on 21 or 22 December. credit: http://bit.ly/1pQn7wy

A common misconception is that seasons are caused by our distance from the sun. At aphelion Earth is 1.0167 Astronomical Units (AUs) or 152,100,527 kilometers (94,510,886 miles) from the sun and during perihelion it is 0.9833 AU or 147,093,163 kilometers ( 91,399,454 miles) from the sun. But the times of greatest solar radiation on a hemisphere happen not because of the distance between the Earth and sun but the tilt of Earth’s rotation axis. These times are during the December and June solstices.

Solar apparent size-perihelion versus aphelion 2012. The red circles show the size of one disk superimposed over the other. Shot by David Dickinson and discussed in http://bit.ly/1pQmrHG

Solar apparent size-perihelion versus aphelion 2012. The red circles show the size of one disk superimposed over the other. Shot by David Dickinson and discussed in http://bit.ly/1pQmrHG

David Dickinson observed the sun with his own telescope during perihelion and aphelion in 2012 to create the above images. They show the sun from Earth on January 4th (perihelion) and July 4th (aphelion.) You can read more about his observations as well as perihelion and aphelion in his July 2, 2013 post on Universe Today.

Perihelion for 2020, the point in Earth’s elliptical orbit when it is closest to the Sun, occurred on January 5th. The distance from the Sun doesn’t determine the seasons, though. Those are governed by the tilt of Earth’s axis of rotation, so January is still winter in the north and summer in southern hemisphere. But it does mean that on January 5 the Sun was at its largest apparent size. This composite neatly compares two pictures of the Sun, both taken from planet Earth with the same telescope and camera. The left half was captured on the date of the 2020 perihelion. The right was recorded only a week before the July 4 date of the 2019 aphelion, the farthest point in Earth’s orbit. Otherwise difficult to notice, the change in the Sun’s apparent diameter between perihelion and aphelion amounts to a little over 3 percent. The 2020 perihelion and the preceding 2019 aphelion correspond to the closest and farthest perihelion and aphelion of the 21st century. Image Credit & Copyright: Ian Griffin (Otago Museum), https://apod.nasa.gov/apod/ap200109.html

Any object orbiting the Sun has a perihelion or aphelion including spacecraft. The Parker Solar Probe has its 7th perihelion on January 17, 2021 and Solar Orbiter has its 1st perihelion on February 16, 2021. 

credit: NASA/ESA/JHAPL/Universe Today/Wikipedia/timeanddate.com