7
$\begingroup$

In the below image, the position of the moon as viewed from the same spot is displayed.
The drawing to the left is from today at local time of 2am.
The drawing to the right is from a couple of months ago at a local time of 10pm.
How is it possible for the moon to be at roughly the same spot with such a difference in time?

enter image description here

$\endgroup$
3
  • 2
    $\begingroup$ Are you asking why the altitude is the same (the angle above the ground) at different times on different days? Are you asking why the azimuth is the same (the direction around the horizon, northeast, east, southeast, etc)? Or are you asking why both the altitude and azimuth are the same? $\endgroup$
    – JohnHoltz
    Aug 3, 2023 at 15:46
  • 1
    $\begingroup$ What phase was the Moon on these occasions? en.wikipedia.org/wiki/Lunar_phase $\endgroup$ Aug 3, 2023 at 19:40
  • 1
    $\begingroup$ I do not remember to be honest. For the current phase of the Moon we can probably check tonight. Anyway, taking this any further is out of my knowledge. Good luck! $\endgroup$
    – Pika-Chu
    Aug 3, 2023 at 19:48

2 Answers 2

20
$\begingroup$

Disregarding the phase, not only is it completely plausible that the Moon is at one position at 10 PM, and the same position at 2AM a couple months later, it is completely plausible that the two pictures could represent observations of the Moon roughly five days apart.

The Moon takes roughly 29.5 days complete a synodic orbital period. The Earth takes on average 24 hours to complete a Solar Day. The combination of these two motions means that every day, the Moon rises about 50 minutes later, and sets about 50 minutes later than it did on the previous day, because the Earth has to turn that much more to put the Moon in approximately the same position for an Earthisde viewer.

So if you see the Moon at some point on the sky at 10PM on the night of day 0, it will be in roughly the same position at 2AM in the morning of Day 6. (though, obviously, the phase will have changed significantly)

When the only the information provided is that the images present views of the moon a couple months apart, without specifying specific dates or phases, any two local times could represent plausible depictions of where the Moon was observed on those unspecified nights or days.

$\endgroup$
7
  • 5
    $\begingroup$ It helps that the period between observations almost exactly straddles a solstice, meaning that the moon traces the same path across the sky, rather than being at a different azimuth. $\endgroup$ Aug 3, 2023 at 12:08
  • 1
    $\begingroup$ Having in mind all that, would the rotation of the Solar system around the black hole it is rotating being clockwise or counterclockwise affect the event of the so called Blue Moon somehow? $\endgroup$
    – Pika-Chu
    Aug 3, 2023 at 17:01
  • 7
    $\begingroup$ That's a separate question, but no. The Solar system isn't orbiting a black hole (the vast majority of the mass in the galaxy is outside of Sagittarius A*), and no, even if it were, it would have nothing to do with blue moons. $\endgroup$
    – notovny
    Aug 3, 2023 at 17:08
  • 1
    $\begingroup$ I said black hole cause i did not know what else to call the thing that the solar system rotates around. Anyway, thank you. $\endgroup$
    – Pika-Chu
    Aug 3, 2023 at 17:39
  • 2
    $\begingroup$ @Pika-Chu Barycentre. $\endgroup$
    – wizzwizz4
    Aug 3, 2023 at 20:08
1
$\begingroup$

In addition to @notovny answer, even if the moon were to be in roughly the same phase, its location at the same time of the day may be notably different.

Actually this happens in the Sun as well - with the most clear-cut example is the Sun Set at seen from the equator at the Summer and Winter solstice (about 6 months apart); So on 21 June and 22 Dec the Sun sets at the same hour but on 21 Jun it sets ~23.5 to the North (i.e., NW) and on 22 Dec it sets ~23.5 degrees to the South. That is because the daily path of the Sun changes a little each day - not only rise and set times.

Similarly, the Moon has those smaller movements in its daily path as well: they are actually 12 times faster (actually even closer to 13 times faster! - about ~27.3 days). So after 6 or say 3 synodic months the Moon will have about the same phase, but it might have quite different daily path.

$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .