If you were entranced by the full Moon last night, and would like to watch it rise again this evening, you’re probably aware you’ll have to wait until later in the night. But how much later? A quick check of moonrise (or set) times over successive nights reveals wide variation – sometimes the difference is just a few minutes from one night to the next, sometimes more than an hour. So what is going on to cause such big disparities?
How moonrise times change
The Moon takes 27.3 days to orbit the Earth. That is not exactly the period from one full Moon to another however, because by the time the Moon gets back to the same location, relative to the Earth, the Earth has moved in its orbit. The Sun is by then in a different spot in our sky and does not illuminate the Moon fully. Consequently, the gap between one full Moon and another is 29.5 days.
One might expect, therefore that the time between moonrises would vary by 24 hours divided by 2.5 days, or approximately 49 minutes.
Yet if you relied on this, perhaps to watch the Moon rise over some landmark, you’d often lead yourself astray.
If you’re in London, the Moon will rise at 10:14pm local time tonight (July 11, 2025). Tomorrow it will rise just 20 minutes later, so if you were planning some dark deeds, or astronomical observations, you won’t have a lot of extra time to fit them in after night fall. You’ll have a little longer the night after, but the gap is actually shrinking – the Moon rises Sunday night at 11:50 pm, so it only squeezes in an extra 16 minutes.
On the other hand, it still needs to get through a 24-hour migration in the course of the month, so on July 25 moonrise is at 5:32 am, followed by 6:37 am the following morning – a jump of 85 minutes. In fact, the gap between moonrises on those two days is larger than that from July 11 to July 16, five days apart.
These days we can not only look up the times of moonrise and moonset with ease, but they don’t affect the lives of most of us much. Things were different a few centuries ago, when the presence or absence of moonlight made a big difference to anyone traveling or hunting by night.
Watching the moonrise in summer can be more pleasant, but the full Moon never gets as high.
Image credit: Wirestock Creators/Shutterstock.com
Why this happens
One reason some people might have guessed for these variations is that the Moon’s orbit is not perfectly circular. That’s why we get “supermoons”, full Moons that coincide with when the Moon is closer to the Earth, and therefore appears up to 12 percent larger and 15 percent brighter.
Kepler’s 2nd law tells us that objects move faster in their orbits when closer to the body they orbit, so the Moon does shift more at some times than others. However, this is only a small factor in the differences described above. The changes we’ve noted also don’t really relate to whether the Moon is full or new, at least unless seasons are taken into account.
If the shift in times was all about how fast the Moon was moving, the differences would be the same, or at least very similar, everywhere on Earth.
However, if we check moonrise at a different location, the picture changes. In Los Angeles, moonrise is 9:13 pm tonight, and 9:48 pm tomorrow, a 35-minute shift compared to London’s 20. On July 26 it rises 66 minutes later than on July 25 – still a much bigger gap than now, but much less than London experiences between those days.
The fact LA is so much closer to the equator than London is a big hint as to why, so let’s try jumping across to the Southern Hemisphere, say Lima.
The Moon will be nearly full as it rises in Peru tonight as well, but it will do it 6:54 pm; only an hour of the difference from the northern cities is caused by daylight saving. Tomorrow, moonrise is at 7:50 pm, a whole 56 minutes later. Between July 26 and 25, the shift will be just 45 minutes.
The pattern is starting to emerge. The shift between rising times day to day (or night to night) is mainly determined by latitude. From Singapore, almost on the equator, the variation from night to night is small. The further you go towards the poles, the bigger it becomes.
That’s because the Moon does not orbit over the Earth’s equator, as simple diagrams may show (and most other planets’ moons do). The Earth’s axis is tilted at 23.4° to the plane in which we orbit the Sun, causing the seasons. The lunar orbit is not perfectly aligned with the plane of the Earth’s orbit, but it’s considerably closer than it is to the equator.
As a consequence, the Moon appears to someone at the equator to shift between a fair way into the southern sky and a similar distance north. At mid-latitudes, the Moon never passes entirely overhead, but its height in the sky can vary a lot. As we noted yesterday, a full Moon in the middle of summer is a great time to observe the Moon Illusion because it stays low in the sky, particularly if you’re as far north as the UK.
If you’re in the Northern Hemisphere the summer full Moon makes a low arc across the southern sky, and the further north you are the lower the arc. In the Southern Hemisphere it’s a northern arc six months later. The winter full Moon, on the other hand comes much closer to passing overhead. When the Moon is new, and therefore close to the Sun in the sky, the low arc occurs in winter.
Traversing a low arc takes less time, just as the Sun is above the horizon for a short time in winter. So whatever season you’re in, when the Moon is near the southernmost part of its cycle, full, new or in between, it won’t be up for long at high northern latitudes. As it moves north, it will spend longer in the sky. Progressively longer arcs mean much later setting times. Unless you are at really high latitudes, moonrise will still be later each day, but not later by much, because that longer cycle partially counteracts the average daily shift. On the other hand, as the Moon makes its way south, a northern observer will see an extended delay between successive moonrises.
All “explainer” articles are confirmed by fact checkers to be correct at time of publishing. Text, images, and links may be edited, removed, or added to at a later date to keep information current.
Source Link: Why Doesn’t Moonrise Shift By The Same Amount Each Night?
