At what times are the planets closest and farthest from the Sun?

Well, I think that we might all recall some teacher somewhere telling us that the Earth is actually closest to the Sun when the Northern Hemisphere is experiencing winter (December 21), and is farthest during the summer months ( June 21) north of the equator. We were all pretty bewildered by this until we learned that the Earth's axis maintains a fixed orientation to the plane of the solar system such that when it is at 'perihelion' at the Winter Solstice, the Earth is tilted away from the Sun, and the slanting of the solar rays cause less light energy to strike the surface at a given time of the day. In the summer, when the Sun is farthest from the Earth at the Summer Solstice, the Earth's axis is tilted towards the Sun, and the light rays angle to the ground at a steeper angle. The Sun rises higher in the sky, and in the northern hemisphere, this all adds up to balmy summer days.

The other planets in the solar system travel around the Sun over different orbital periods. Their years, measured in Earth days, are very different than our own. Mercury takes 87 Earth days to go once around the sun, Jupiter takes 11.8 Earth years, and Neptune takes 164.8 Earth years. The question of when will each of the planets be closest ( perihelion) and farthest ( aphelion ) from the Sun will be different for each planet. There is no fixed time during a single EARTH year when this will happen, but instead you have to work out the locations of all the planets in their orbits and determine their perihelia and aphelia. From this you can find the appropriate Earth day, month and year when this will happen.

The easiest way I know how to do this is to go to the US Nautical Ephemeris, which is published every year, and check out their tables for the heliocentric distance of the planet from the Sun, usually given in Astronomical Units ( ie in units of the radius of the Earth's orbit of 93 million miles). You then scan the table for the date when this number is a minimum ( perihelion) and a maximum ( aphelion). Since Venus and Mercury orbit the Sun in less than one year, you can just use the 1995 or 1996 Ephemeris to look this up. For the rest of the planets, you will need to go back a bit further. It's a real bear of a process.

A shortcut is to find the current heliocentric longitude of the planet, and its change per day, and compare this with the orbital element of the planet's 'Longitude of Perihelion'. Example, For Mercury, on January 3, 1995 its longitude was 345.7 degrees, and its Longitude of Perihelion is 77.37 degrees. The planets daily longitude changes by 4.09 degrees per day, so this means that its next perihelion will occur when the planet has moved an additional 77.37 + (360.0 - 345.7) = 91.67 degrees. At a rate of 4.09 degrees/day, this will take 22.41 days from January 3, 1995 so that its next perihelion will be on January 25, 1995. Since Mercury goes once around the sun every 87.6 days, it will reach aphelion after 87.6/2 = 43.8 days after perihelion passage on January 25. Below I will list the upcoming, next perihelia and aphelia for the planets.

                 Distance (mega miles)              Next Date
              Perihelion     Aphelion       Perihelion      Aphelion
  Mercury       28.6           43.4          10/16/1995      11/29/1995
  Venus         66.8           67.7          8/11/1995       12/1/1995
  Earth         91.4           94.5          12/21/1995       6/21/1996
  Mars         128.4          154.9          2/19/1996        1/28/1997 
  Jupiter      460.3          507.2          5/5/1999         3/29/2005
  Saturn       837.6          936.2          5/26/2003        2/8/2018
  Uranus      1699.0         1868.0          3/1/2050         4/17/2008
  Neptune     2771.0         2819.0          3/2030           2/2112
  Pluto       2756.0         4555.0          8/1989           8/2113
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Unless I have made some dumb mistake in adding and converting from elapsed days to a calendar date, these dates should be more or less correct. Jupiter's last Aphelion was on June 12, 1993, and Saturn's was on September 16, 1988. Uranus was at perihelion on June 4, 
1966. 







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