Over the last few days, here in Eastern Canada, the moon (Full and close to it) has been appearing very large in the atmosphere around sunset. I know that also happens at around 2->3:00 am in the summer.

How can I figure out when such phenomena will happen?

  • 2
    The search term would be "Lunar Perigee", there are many calculators online. Physical distance accounts for only 12% of the difference. The rest is due to the illustration that a moon near the horizon usually appears larger because it is not lost in the sea of the sky.
    – Phil
    Jan 29, 2013 at 21:11

4 Answers 4


As previously mentioned, The Photographers Ephemeris is an awesome program and what I personally use. I highly suggest viewing on a tablet versus a phone.

Also, the whole moon appearing larger thing is an optical illusion during its rising and setting. The moon doesn't actually get closer to the Earth, in fact, it's progressively moving further away day-by-day.

The illusion of largeness (official industry term) is basically a result of there being foreground stuff (e.g. trees, buildings, etc.) that makes it seem larger.


Lastly, I suggest you use a telephoto to help magnify the overall size of the Moon while still framing it with some sort of foreground object to make it more interesting.

  • "The moon doesn't actually get closer to the Earth, in fact, it's progressively moving further away day-by-day" - Of course its distance changes throughout the year; that's why its orbit is known as 'elliptical' rather than 'circular.' Here's a picture of the orbit. See also wikipedia: "The non-circular form of the lunar orbit causes variations in the Moon's [..] apparent size as it moves towards and away from [..] Earth." Also, the orbit's radius is indeed increasing, but only by a few millimeters a day... Jan 30, 2013 at 2:33
  • I stand corrected. Apologies for the inaccuracy of my statement about the whole getting closer thing, and thanks for the correction! My context was of course in the more grander scheme of things where, eventually, our Moon will spiral away from us just like in the T.V. show Space 1999 =] Jan 30, 2013 at 15:16
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    @ISO's core point that the distance to the moon has very little to do with its apparent size is correct, though.
    – Reid
    Feb 1, 2013 at 18:13

I use the Photographers Ephemeris. The great thing is that they have portable versions for mobile devices so you can take it with you to the location or for travel. Here is a description from their website:

The Photographer’s Ephemeris (TPE) helps you plan outdoor photography shoots in natural light, particularly landscape and urban scenes. It’s a map-centric sun and moon calculator: see how the light will fall on the land, day or night, for any location on earth.

It obviously does much more then just predict the moon size.

  • The apparent size is an optical illusion, as @ISO notes in the other answer. Distance to the moon is a 2nd- or 3rd-order effect.
    – Reid
    Feb 1, 2013 at 18:14
  • @Reid - Yes you are correct. I just use the tool I noted to determine where the moon will be, and then you can use the optical illusion to your advantage with that knowledge!
    – dpollitt
    Feb 1, 2013 at 20:33

Use a calculator:


The difference in-camera is not massive, but noticeable.


In addition to the suggestions already given, a visual indication of size is available with the calendar at www.paulcarlisle.net/mooncalendar. The Settings icon lets you adjust the scaling between none, normal, and exaggerated. You can quickly see when the largest apparent moon in a given month occurs.

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