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I'm looking at the second edition of Meeus's "Astronomical Algorithms," chapter 47, "Position of the Moon." At the bottom of page 337, right before getting into the actual formulae of the algorithm, Meeus states

Moon's mean longitude, referred to the mean equinox of date, and including the constant term of the effect of the light-time (-0".70):

The size of the correction corresponds with the light travel time between Earth and the Moon and the Moon's orbital angular velocity, so it's clear to me that it's correcting for the light travel time between these two bodies. My question is, which direction is the correction? Is this converting from an inertial calculation to the apparent Moon position as would be determined by an observer on Earth, where they would see the Moon at the given time (lagging a bit due to the light travel time)? Or is this correcting from a calculation of the apparent location to the true/inertial location?

Phrased another way, is the Moon location algorithm in Meeus providing a true/inertial location (what the Moon would say its location is), or is it providing the apparent position as viewed from Earth, corrected "backwards in time" to account for the light travel time between the two bodies?

NeutronStar
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  • I'm not familiar with "inertial place", the Explanatory Supplement defines several "places". I don't think the intermediate positions in Meeus fall directly into any of them. Applying light time correction moves closer to the "apparent place", but there's still quite a few more corrections needed to get there. Meeus has most of those corrections in the chapter titled "Apparent Place of a Star". – Greg Miller Mar 12 '24 at 16:21
  • You will likely have more luck at the Astronomy stack exchange. – Organic Marble Mar 12 '24 at 17:02
  • That's a really, really small correction... – Darth Pseudonym Mar 12 '24 at 20:51
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    @DarthPseudonym At the distance of the Moon, it's a bit more than a kilometer. That's small for some applications and large for others. (And it's smaller than Meeus's stated accuracy of the algorithm, 10" in longitude and 4" in latitude.) But that doesn't change my question. – NeutronStar Mar 12 '24 at 21:09
  • @DarthPseudonym when laser beams are bounced off of the retroreflectors on the Moon for ranging purposes, it's a significant correction. See for example How does a laser from Earth manage to hit the Moon with precision? and to a lesser extent, When they shoot lasers at the Moon for ranging, what is the shape of the beam? – uhoh Mar 13 '24 at 20:32
  • @NeutronStar the problem with comments like OgranicMarble's is that they don't remind you that you should not maintain two identical copies of a question open on different sites at the same time. Cross-posting is discouraged because (among other things) it leads to answer fragmentation. You should probably delete one of them, or at least modify one so that it asks a similar but recognizably different question. – uhoh Mar 13 '24 at 20:36

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