Moonlight Versus Starry Night Skies

The light from the full moon is about 135 times brighter than starlight! ~ © Royce Bair

Even a quarter moon, low in the sky, can overpower all but the brightest stars ~ © Royce Bair

Even a crescent moon is 5X brighter than starlight.

Avoid Moonlight for the Best Starry Night Skies: Because moonlight is about 135 times brighter than starlight, your best starry night skies will be when the moon is not present. Even a crescent moon can be about five times brighter than starlight.

Only on a new moon? It would seem then that only on the night of a new moon, or the first night of a waxing moon, or the last night of a waning moon would be the only three nights of the month available for good star photo photography! Not so. While these three nights do offer excellent night darkness for star photography, there are selective hours on many other nights of the month that are suitable for good star photography, as you will see from the chart below. (To track the phases of the moon, I use The Old Farmer's Almanac Moon Phase Calendar.)

The day of the New Moon, and the day before & after offer excellent night darkness for star photography.

Nights before the moon rises, or after it sets: There are also those periods of the night when the moon has not yet risen, or it has set. These periods can often give you several hours each night for great star photography.

This window of photo opportunity must also meet the twilight requirement of being at least two hours after sunset, and at least two hours before sunrise. Let me explain, using the U.S. Naval Observatory charts for moonrise and moonset in conjunction with their charts for sunrise and sunset. With the U.S. Naval Observatory website you can enter a location and obtain a table of moonrise/moonset and a table of sunrise/sunset times for the whole year of your choosing.

Moonrise/set & sunrise/set times for July 2013 in Moab, Utah

Using moonrise/moonset charts: In this custom chart, I've taken moonrise/set times for the month of July 2013 and combined them next to the sunrise/set times for the same month (this illustration is for Moab, UT USA, which is the closest city to Arches National Park). On July 1st, the moon rises at 00:27 (12:27A.M.) and sets at 14:07 (2:07 P.M.) The sun rises at 04:58 and sets at 19:46. The 4th column shows the window of photographic opportunity. On July 1st, at 2 hours after sunset (21:46) you can start taking the best star photos, and continue until the moon rises the following morning at 01:01 (taken from the July 2nd moonrise row and column). The total time available for star photography (5th column) is 3:15 (3 hours 15 minutes). On July 4th, the window of opportunity begins to be limited by the rising of the sun, not the moon (02:59 is two hours before the sun rises at 04:59).

The area highlighted in darker blue is the 3-day period in July around the New Moon. Normally, this period would lengthen your night opportunity, but the long days and short nights of summer are the limiting factor here. From July 15 - 29, the moon goes from its First Quarter (50%), to Full Moon (100%), and to it's Last Quarter (50%). During this very bright period, it can still give off quite a bit of light to the sky, even below the horizon —for up to 30 minutes after it has set, and for up to 30 minutes before it rises. Hence, the highlighted times in the "Window of Opportunity" column. The sky directly above the setting and rising moon can be affected during these time periods, similar to this image.

NOTE: *These blank spaces in the table indicate that a rising or a setting did not occur during that 24-hr. interval. **During the "Window of [Photographic] Opportunity" I have set that period to midnight (00:00), even though it would most likely fall a few minutes after midnight on the following day. Please note that these times are shown in Mountain Standard Time (if you actually use this chart, you'll need to add one hour for Daylight Time, which would be in effect during July).

Starry Night Goal: If you follow these guidelines in avoiding moonlight, twilight from the sun, and man-made light pollution, your reward can be shots like this:

Milky Way over Teton Range and Jackson Lake ~ © Royce Bair

Royce Bair is the editor of this blog and the photographer of the above images. Here is my gallery of NightScape images. My schedule of workshops, tutorials, and other events is available here.

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NASA's Dial-A-Moon Tool

Last week, NASA released an incredible video showing the Moon’s motion and phases over the whole year of 2013, where each frame of the video is one hour of time! It’s mesmerizing:

If you have the bandwidth, change the resolution to full, 1080p HD (using the cog icon at the bottom right) -- the extra quality is worth the watch!

The animation uses data from the Clementine lunar mission, a joint NASA and military probe in the 1990s to map the Moon, as well as images and topography measurements from the Lunar Reconnaissance Orbiter. The surface-elevation measurements were used to create the realistic shadows in the animation.

You'll noticed that the Moon appears to rock around, tipping, and tilting over the course of every month. That’s called libration, and is due to the Moon’s orbit being elliptical, as well as the Moon's rotational axis being slightly tilted with respect to its orbit.

There are many other extras to this video, like the scale on the right showing how far the Moon is from the Earth in units of the Earth’s diameter (about 13,000 kilometers, or 8,000 miles). You can watch it oscillate back and forth as it orbits, moving from 28 to 32 times the Earth’s diameter away.

At the bottom right is a huge amount of information that changes with time, including the Moon’s distance and its apparent size, measured in arcseconds. There are 3,600 arcseconds in a degree, and a full Moon is roughly 1,800 arcseconds, or a half degree across.

DIAL-A-MOON Tool: Although the video is fun to watch, NASA's website for the animation may be more useful for night photographers. This site contains all the individual frames used to animate the Moon phases and libration. Their Dial-A-Moon tool will allow you to pick any month, day, and hour of the year and see that phase of the moon (a lot easier than trying to adjust the video)!

Rising and Setting Times: Knowing the phase of the Moon is great, but a night photographer also needs to know the rising and setting times of the moon on any given day. For that you'll also need this USNO table. This table can be changed to show the rising and setting of either the sun or the Moon.

Position of the Moon: Wouldn't it also be nice to know where the Moon would be in the sky on any night and hour? The free planetarium software, Stellarium, can do this for you. All you do is tell it where you are on the earth and what time of the day or night. You can advance or go back to any day, hour, minute, and second in time (you can also speed up time and watch the sky move quickly or slowly). Using the coordinates and time you give Stellarium, you'll be able to see exactly where the moon rises and sets, or where it is in the sky at any hour (as well as all the stars, constellations, and the Milky Way)!