perihelion & lowest light-time don't coincide

[MonDie]

I set the HORIZONS interface to give the "one-way (down-leg) light-time" of the sun. It reaches its minimum at 7:00am UTC/GMT, but most sources assign perihelion to 12:00 or 11:00am UTC/GMT. What's the discrepancy from?

Edited January 4, 2015 by MonDie

[Harold Squared]

Could refraction by the atmosphere be involved? And am I wrong thinking perihelion is more of a yearly phenomenon than a daily one?

[swansont]

Perihelion is definitely an annual phenomenon, since the earth orbits once per year.

[Harold Squared]

Might the confusion stem from observer vs object to be observed having different perihelion values?

[Janus]

I think I might have an answer. The perihelion time is likely given for the Earth-Moon barycenter. The Earth orbits around this barycenter (which is just a little under the surface of the Earth), which means that it is sometimes a bit closer or further from the Sun depending on the the phase of the Moon. As a result, the Earth's distance from the Sun at any given moment is a combination of the Moon-Earth barycenter distance and the Earth's position with respect to the barycenter due to Moon phase.

 

 

Now it turns out, that Jan 4, the day of perihelion, is also the day of a full moon. This puts the Earth on the "sun-side" of the barycenter.

 

So, this just might mean that due to this in and out motion due to the Earth-moon orbital action, the Earth could be at its closest to the Sun at a slightly different time than when the Earth-Moon barycenter is at its perihelion.

[MonDie]

The Horizons interface isn't work from this platform, or they changed something. Janus, I considered your response. I've never studied astronomy heavily, and I'm not sure whether to expect a preceding full moon or a succeeding full moon. I expect that the orbital calculation would be simplest tracing the earth-moon barycenter, but this is the three-body problem after all. If I'm right, the full moon should push the earth out toward the sun, causing the lowest light-time to fall somewhere between perihelion and the full moon. Given that the lowest light-time preceded perihelion, I would expect a preceding full moon. Did the full moon precede perihelion? I can't check.

 

 

Perihelion is definitely an annual phenomenon, since the earth orbits once per year.

 

I thought the date would be implied since I was posting on the day of the event, excluding the timezones where it was about bed-time anyway.

Edited February 28, 2015 by MonDie

[swansont]

 

I thought the date would be implied since I was posting on the day of the event, excluding the timezones where it was about bed-time anyway.

 

My comment was addressed to H², who asked if it was a daily phenomenon.

[MonDie]

The full moon was about nooonish (UTC/GMT) on January 3rd, the day preceding. Looks good, Janus.

Edited March 2, 2015 by MonDie