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RASC Calgary Centre - Sunset and Moonrise

By: Larry McNish
Page last updated: January 3, 2017
(Page originally created March 28, 2007)
(Diagrams on this page are by the author)

Does Moonrise Ever Occur Exactly at Sunset?

Answer:

The answer is yes - the Moon rising when the Sun is setting is actually quite a common occurrence.

However, the time of Sunset changes throughout the year, getting later in the Summer months and earlier in the Winter months. The time of Moonrise depends on the geometry of the Sun, the Earth in its orbit around the Sun, and the Moon in its orbit around the Earth. So, is there ever a time when these two events happen at the same time for a given location on the Earth?

Yes - Every Full Moon the Sun, Earth and Moon line up such that people on the Earth (between the Sun and the Moon) see the Sun setting in the west and the Moon rising in the East at the roughly the same time:



For example, at Calgary on March 3, 2007 - the Sun set at 18:20 MST and the Moon rose at 18:19 MST - within a minute of each other.

Since the setting and rising times are defined as when the upper limb of the objects appear at the horizon, and since the horizon is never really flat, and since no-one is ever really at "sea level", and since the atmosphere near the horizon bends light so that they appear flattened and higher than they really are - this is as close to "simultaneous" as it gets. (This is why the rising and setting times of the Sun and Moon are usually specified as the closest minute, not to a fraction of a second.)

On April 2, 2007 they occur about half an hour apart.
On May 1, 2007 they occur about 7 minutes apart.
On Sept 27, 2007 they occur at almost exactly the same time.

If you go to the DarkSky calculator page you can enter your exact latitude and longitude and get a chart of the events on every day of the month for your location.
Note that the chart lists Standard Times only - add an hour to all events during DST.

The graph below shows the Time of Day across the top and the Dates of the Year down the left side.

The time of Sunset at Calgary for each day of the year is shown as the yellow curved line - getting later in the summer months.

The time of Moonrise for Calgary is shown as a gray dot for each day of the year.

Where the gray dots overlay the yellow line, the two events are happening at roughly the same time - i.e. the time of the Full Moon.

As you can see there are 6 dates in 2007 when the Full Moon rising and the Sun setting happen very close together, but every Full Moon they are pretty close.



This graph also shows something else. Note the time of Moonrise on the days at the end of September.

The "Harvest Moon" is the full moon closest to the Autumnal Equinox, which occurs on or about September 23. In 2007, the September Full Moon occurs on September 27. Note that surrounding that date, the Moon is always rising within an hour or so of sunset. The Full Moon (or nearly Full Moon) on these dates provides extra light in the early evenings after sunset to bring in the harvest - hence the name.

This close grouping of Moonrise times then drifts away from the Sunset line until the following year.


So, what's with the crazy shape the Moonrise bullets make on the graph?.
Why doesn't the time of Moonrise stay the same each day or at least vary by the same amount from day to day?

That answer is considerably more complicated. However it is based on the following:
  • the Earth is zipping along in its orbit and the Moon is circling us at its own pace
  • the Earth's orbit is an ellipse - that means we are moving faster at some points in our orbit and slower at other points during the year
  • the Moon's orbit is an ellipse - so "ditto" - but it's speed is changing during every month as well as its distance
  • the Moon's orbit is tilted 5.16° with respect to the Earth's orbit so its fast points and slow points don't line up every month
  • the Earth is tilted 23.4393° with respect to its orbit
  • our location on the Earth determines the angles at which we see the Ecliptic and therefore the Moon's motion along its orbit
  • the average Lunar Month is 29.530589 days which doesn't divide evenly into our average 365.2425 day year.
  • all this whirling, slowing down, speeding up and tilting causes the closely grouped time of Moonrise to occur at different phases of the moon and at different times of the month. It also affects where on the horizon the Moon appears, as well as how high or low it appears in the sky.
  • it takes 19 years for the cycle to repeat almost exactly (so the same events occur on the same dates). (19 years x 365.2425 days/year = 6939.6075 days, and 6939.6075 days / 29.530589 days per Lunar Month = 234.997 or almost exactly 235 Lunar Months. This period of 19 years is called a Metonic Cycle and was part of the basis for many ancient calendars.
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