Lunar Orbit Issues & Lunar Clocks

[Jaziel]

TL;DR

Conflict between the sidereal and synodic months regarding a particular lunar clock.

I recently purchased a lunar clock intended as a Christmas present for a young family friend who has become interested in astronomy. The clock face shows twelve images of the Moon in all its main phases, with the Full Moon depicted at the "twelve o'clock" position. Now I purchased this clock especially early to allow time to check on its accuracy. After waiting for a full lunar orbit to occur, I have since discovered that the clock time when measured relative to its lunar images (that's to say from New Moon back to New Moon) is slow by a factor of 56 hours.

What is confusing the issue is whether this shortfall of 56 hours is due to the difference in time between the sidereal month (27.2 days) and the synodic month (29.5 days) - or whether there really is a fault with the clock's movement. All I can be certain of is that by adding the 56 hours to the sidereal month produces the following result: 56+652.8 = 708.8 hrs = 29.53 days. This then (approximately) equals the synodic month, yes? If so, why doesn't this result tally with the lunar phases depicted on the clock face. The reason I'm asking this question is because the most recent Last Quarter occurred at 20:51 UTC on the 11/12/25 (World dating format) wasn't actually achieved by the clock until noon the following day (12/12/25) - here a discrepancy of some 14 hours!

It's this issue that bugs me most of all - the marked difference between what is shown on the clock face and the actual lunar phases flagged up by reputable astro websites like Time & Date. This leaves me wondering what would happen if I simply let matters ride? What will these discrepancies add up to in another six months, for example? The mind boggles.

 

[DaveC426913]

TL;DR: Conflict between the sidereal and synodic months regarding a particular lunar clock.

I recently purchased a lunar clock intended as a Christmas present for a young family friend who has become interested in astronomy. The clock face shows twelve images of the Moon in all its main phases, with the Full Moon depicted at the "twelve o'clock" position. Now I purchased this clock especially early to allow time to check on its accuracy. After waiting for a full lunar orbit to occur, I have since discovered that the clock time when measured relative to its lunar images (that's to say from New Moon back to New Moon) is slow by a factor of 56 hours.

What is confusing the issue is whether this shortfall of 56 hours is due to the difference in time between the sidereal month (27.2 days) and the synodic month (29.5 days) - or whether there really is a fault with the clock's movement. All I can be certain of is that by adding the 56 hours to the sidereal month produces the following result: 56+652.8 = 708.8 hrs = 29.53 days. This then (approximately) equals the synodic month, yes? If so, why doesn't this result tally with the lunar phases depicted on the clock face. The reason I'm asking this question is because the most recent Last Quarter occurred at 20:51 UTC on the 11/12/25 (World dating format) wasn't actually achieved by the clock until noon the following day (12/12/25) - here a discrepancy of some 14 hours!

It's this issue that bugs me most of all - the marked difference between what is shown on the clock face and the actual lunar phases flagged up by reputable astro websites like Time & Date. This leaves me wondering what would happen if I simply let matters ride? What will these discrepancies add up to in another six months, for example? The mind boggles.

Can you boost the clock's voltage by (27.2/29.5)%?

 

[Jaziel]

Unfortunately, no. This battery powered clock (which wasn't cheap) does include a knurled knob at the back, enabling the 'hand' or pointer to be adjusted, but that's it, I'm afraid.

Personally I strongly suspect the clock's mechanism to be flawed in some way, but don't have anything like enough technical understanding of the Moon's orbital movements as these apply to the clock's own motions to defend this position. I'm sorry if this comes across as confusing, but this is because I'm right out of my depth here.

 

[Jaziel]

PS. Good news! I've just contacted the supplier and they confirm that the clock is definitely at fault and that they're perfectly happy to give me a refund upon returning it to them, which is a mighty relief, I must say. But thanks for your input, DaveC426913!

 

[Baluncore]

The clock designer appears to have used the sidereal month, not the synodic month, in their design calculations.
Sidereal month = 27.32166 days.
Sidereal year = 365.25636 days.
Synodic month = 29.53059 days. (Phase of the Moon).
The periods are related by: 1 / 27.32166 - 1 / 365.256 = 1 / 29.53059