Earth's Perihelion

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david starling
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Earth's Perihelion

Post by david starling » Fri Mar 08, 2019 4:36 am

Is there a website that lists the location of Earth's Perihelion in the coming years? I know it has Direct-motion relative to the constellations, and is currently mid-Sagittarius, but I need exact data.

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Jim Eshelman
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Re: Earth's Perihelion

Post by Jim Eshelman » Fri Mar 08, 2019 9:49 am

It's amazing that this information is hard to find, and especially that it isn't a retrievable detail in Solar Fire. I used astrology software decades ago where this was routinely available, but it seems to have fallen away.

Here is a site that lists the exact time Earth reaches perihelion for the next several years, so you can calculate Sun's position:
https://www.timeanddate.com/astronomy/p ... stice.html

Oh, this one is better. It gives perihelion and aphelion dates and times for the next century.
http://www.astropixels.com/ephemeris/perap2001.html

In January 3, 2019 the perihelion, therefore, had Earth heliocentrically at 17°31'15" Gemini. Ten years later, on January 2, 2029, it will be at 17°29'41" Gemini, about a minute and a half different,
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david starling
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Re: Earth's Perihelion

Post by david starling » Sat Mar 09, 2019 12:46 am

Thanks Jim. As I understand it, nutation, due to the Moon's orbital effect, causes a +/- difference of about 2 degrees from the Mean position. It seems to jump around without a pattern. So the True location can move as much as 2 degrees either way from one year to the next. Do you happen to know if there's a predictable nutation cycle? That way, one could calculate the Perihelion from one year to the next without having to re-calculate the Sun's position each year.

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Jim Eshelman
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Re: Earth's Perihelion

Post by Jim Eshelman » Sat Mar 09, 2019 8:19 am

david starling wrote:
Sat Mar 09, 2019 12:46 am
Thanks Jim. As I understand it, nutation, due to the Moon's orbital effect, causes a +/- difference of about 2 degrees from the Mean position. It seems to jump around without a pattern. So the True location can move as much as 2 degrees either way from one year to the next. Do you happen to know if there's a predictable nutation cycle? That way, one could calculate the Perihelion from one year to the next without having to re-calculate the Sun's position each year.
It's obviously a computable value since historically it's been available in software, published in nautical almanacs, etc. One easily detectable mark of nutation, though, is that it would vary most within the year. I suggest calculating five years' worth from the site I gave above including aphelion. you can see how linear it is this way. - How precisely do you need it (second? minute?)?
Jim Eshelman
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david starling
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Re: Earth's Perihelion

Post by david starling » Sun Mar 10, 2019 9:15 am

Just to the minute. Good tip about the aphelion!

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