Five Useful Numbers for Sun and Moon Photographers


If you photograph outdoors, chances are you shoot the sun or the moon. Here are five useful numbers that can help plan your shot to best advantage.

29.5 Days: the Lunar Cycle

The cycle of the moon – from new moon to new moon – lasts 29.5 days on average. Once a month, you have the opportunity to photograph the moon in each of its phases!

The moon is tricky: the average calendar month lasts 30.4 days, so the moon’s 29.5 day cycle means that the lunar cycle moves one day sooner each month. One effect is that on some days the moon either does not rise or does not set. And from time to time – once in a blue moon, in fact – there are four full moons in a season, rather than three.

Different phases of the moon present different photographic opportunities. At full moon, the sun’s rays strike perpendicular to its surface, washing out interesting detail on the moon’s surface. Compare that with the quarter moon, where side lighting allows even a modest telephoto lens to show the topography of the moon to good effect.

The Quarter Moon

Since cameras have a limited dynamic range, it is best to control contrast by shooting at twilight or when the sun is still up, so you can include landscape features or city skylines in your shot.

You can use TPE to see which months have the full moon setting or rising during twilight. The day before full moon is often best to shoot the rising moon in the east around sunset. The day after full moon is often best to catch it setting in the west in the morning around sunrise.

15 Hours: the Youngest New Moon

The instant of new moon cannot be seen with the naked eye: the moon is incredibly thin and lies close to the sun in daylight at this time. The youngest new moon ever sighted by the naked eye was 15 hours 32 minutes old.

The moon has actually been photographed at the precise moment of new moon, using an array of special equipment; it is an amazing achievement, but perhaps not the most visually striking photograph in terms of scenic beauty.

Consequently, the exact time of new moon is of limited value. If you want to photograph the new moon without special equipment, you need to know when it will first become visible. This can be one, two or even three days after the stated new moon time – and it certainly will be a minimum of 15 hours after.

TPE uses a method developed by Dr. Bernard Yallop of Her Majesty’s Nautical Almanac Office to predict when to sight the new crescent moon: plan your shoot around the best time shown by TPE for the first day when the crescent moon becomes visible (over two full days after new moon, in the screenshot below):

Crescent Moon Visibility

14% Larger: Supermoon

The phenomenon of “supermoon” has caught the public’s attention in recent years. It was widely reported that the supermoon of 6 May 2012 was “14% bigger”.

14% bigger than what?

The moon’s journey around Earth is not circular. Through the lunar cycle, the moon moves closer and farther from Earth. Combine that with the elliptical orbit of the Earth around the sun and you have something that doesn’t look as quite neat as the solar system models we all built at school.

Supermoon is the name given to the full moon at perigee. Perigee is the point during the lunar cycle when the moon makes its closest approach to Earth. Apogee is when the moon is most distant from Earth.

Moon at Perigee

The 14% figure is the observed width of the perigee moon compared to the apogee moon. 14% doesn’t sound much, so does it actually matter to photographers?

For telephoto shots, yes.

A 14% increase in diameter corresponds to a 30% increase in area. For example, shooting with a Nikon D300 and a 500mm lens, supermoon will be around 880 by 880 pixels in your shot. An apogee full moon will only cover around 770 by 770 pixels (see diagram below).

0.5°: the Half-Degree Rule

As seen from Earth, the sun and moon both occupy a width of approximately 0.5° in the sky.

The half-degree rule is a rule of thumb: both sun and moon in fact vary in observed size over time. The sun increases from 0.52° at aphelion in June to 0.54° at perihelion in December. The moon varies even more: from 0.48° to 0.56°.

The solar eclipse of May 20 2012 was an Annular eclipse: the moon was further from Earth and did not fully mask the sun, resulting in that striking ring-of-fire appearance.

When photographing the sun or moon the half-degree rule can help with your shot planning. If you know the moon is approximately half a degree in diameter, and want it to occupy a certain fraction of the frame, you can calculate the camera and lens combination you’ll need.

A 35mm full frame camera with a 200mm lens has a field of view of 10° across in landscape orientation. This means that the moon at 0.5° diameter will only occupy 1/20th the width of the frame (0.5° divided by 10°). By extension, if we doubled the focal length of the lens to 400mm the moon will still only occupy 1/10th of the frame.

A camera with a smaller sensor, say a crop factor of x1.6, and a 500mm lens gives a field of view of 3.4°. The moon would then occupy one fifth of the frame – much better!

Perigee vs Apogee

4 Minutes per Degree: Earth’s Rotation

The Earth takes four minutes to rotate by one degree. How do we know this?

1 day = 24 hours = 1440 minutes
1 day = one rotation of the earth = 360°
1440 minutes / 360° = 4 minutes/degree

We can use this information to gain a sense of how quickly the sun or moon will move in the sky. Using the half-degree rule, we can quickly determine that they will take around two minutes to move by one diameter.

This gives a pretty good idea of what is going to happen in those frenzied photographic moments just before sunset or moonset!

Hold your hand at arm’s length and clasp the moon between your thumb and forefinger. That’s half a degree. Now, still with your hand at arm’s length, take that distance and count how many times you can fit it in between the bottom of the moon and the horizon. Is it three times? You’ve got around six minutes left before moonset. You’d better stop reading this and take the photograph!

(Warning: never look directly at the sun. Doing so can cause permanent damage to your eyesight.)

Once you begin to use the size of the sun and moon as a yardstick, it’s easy to develop an instinctive sense of their movement. For outdoor photographers, this is invaluable when estimating how long before the sun will disappear behind a ridge or building without having to pull the smartphone out!

Remember, though: the sun and moon don’t just move vertically. It’s important to account for lateral movement too: for example, the low winter sun skims along the horizon. TPE can help you understand and plan for this.

Knowing how fast the moon is moving in the sky is also useful when planning longer exposures or multiple exposures.

There’s a practical limit to how long a successful exposure of the moon can be. As we know, the body will move by its own width in around 2 minutes. That means a 30 second exposure of the moon will see it smeared across the frame by one quarter its diameter: you need a much faster shutter speed!

When setting up multiple exposure shots of the moon, anything less than two minutes between exposures and the moon will overlap. Four-minute intervals will give a better spacing.

If you’ve enjoyed this post, you might also enjoy “Understanding Light with The Photographer’s Ephemeris” co-authored with renowned landscape photographer Bruce Percy. It’s available through Bruce’s web-site.