My textbook (and lots of other sources) shows the earth's longitude of perihelion at the epoch around 103 degrees. I understand that this means (in the neighborhood of J2000) the earth will reach perihelion about 103 degrees after the vernal equinox, however, this would be very close to aphelion (July 6ish) not perihelion (January 3ish).

Put another way: If the vernal equinox is March 20th & perihelion is January 3rd, shouldn't the longitude of perihelion be about -76 degrees (or 284 degrees)?

Help! What am I missing?

ANSWER--Credit to @Mike_G below, restated here because the final answer is in the comments: "At the March equinox, the alignment is Earth - Sun - first point of Aries." Beginner mistake: I was trying to find the earth's position in time but I thought the March equinox was when the earth was between the Sun & first point of Aries, which is the wrong arrangement.


1 Answer 1


The Earth's heliocentric ecliptic longitude is opposite to the Sun's geocentric ecliptic longitude. If the Earth's perihelion is at 103 degrees heliocentric longitude, then the geocentric longitude of the Sun at that time is 283 degrees, or 13 degrees past the December solstice.

  • $\begingroup$ Thanks @Mike_G. I wasn't trying to calculate geocentric coordinates yet, but I think you pushed me into an answer. Can you answer this followup: If I take a picture of the solar system from above the north pole at the vernal equinox, is the x-axis lined up as earth-sun-aries, NOT as sun-earth-aries? I think I had been using the wrong arrangement. Hope you'll forgive me--I'm new to this an live in a cloudy environment, spending more time with equations than looking at the sky! $\endgroup$
    – MrPat
    Apr 18, 2018 at 6:14
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    $\begingroup$ At the March equinox, the alignment is Earth - Sun - first point of Aries. $\endgroup$
    – Mike G
    Apr 18, 2018 at 7:02

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