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When Does the Earth’s Prime Meridian Line Up With the Celestial Meridian?

-- | August 1, 2015

Question: On what date and time to the nearest second (in the past) has earths prime meridian directly lined up and faced the “celestial meridian” and I am talking about using the “equatorial coordinate system”

Definition for “Celestial Meridian” which I am using from Dictionary.com below:
“In the equatorial coordinate system, a great circle on the celestial sphere passing through the celestial poles and the vernal equinox. It represents the zero point for the horizontal coordinate in this system, having a right ascension of 0 hours.”

I might have thought it was vernal equinox but that happens once every year but the earth doesnt rotate exactly 360 degrees per year so the prime meridian on earth would not always line up. So thats not my answer so I cant just look up Vernal equinox dates on the web.
Its might happen every 4 vernal equinox but not sure. That still would not give me a date which I am looking for.

Prefer UT time or Julian date but anything would help.
Does the date time I am looking for even have a unique name for it?

— Steve

Answer: The Earth’s “prime meridian” is the great circle which passes through the Earth’s poles and represents the start of the longitude coordinate system, or zero-degrees longitude. Comparing that to the definition you have given above for the celestial meridian, I would say that these two meridians line up once each day. Note, though, that the two prime meridians are exactly parallel only twice each year; on the vernal and autumnal equinoxes. The exact time in a day that the prime meridian aligns with the celestial meridian is determined by the difference between a solar and sidereal day, or about 4 minutes.

Jeff Mangum

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