See The Supermoon On Saturday

You’ll probably be hearing a lot about a “super moon” for the next few months. This is not a term that astronomers use, but here are some facts about what will actually be happening.

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Full moon is when the sun, Earth, and moon line up with the Earth in the middle. As seen from the surface of the Earth, the moon is fully illuminated. Because it is exactly opposite the sun in the sky, the moon rises in the east just as the sun sets in the west and, roughly 12 hours later, sets in the west just as the sun is rising in the east.

Because the Earth is constantly revolving around the sun and the moon is constantly revolving around the Earth, full moon is an instantaneous event, occurring when the moon is exactly opposite the sun. This week this happens at 2:35 p.m. EDT on Saturday, August 29. 

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Full moon occurs this month at 2:35 p.m. EDT on Saturday, August 29. Credit: Starry Night software.

A minute earlier, the moon’s phase is “waxing gibbous,” and a minute later it is “waning gibbous.”

Each full moon occurs roughly 29.53 days after the previous full moon. “Roughly” because the moon’s orbit around the Earth is not a perfect circle, but instead is elliptical in shape. The exact time of full moon varies a little bit from month to month.

The most important result of the moon’s elliptical orbit is that sometimes the moon is closer to the Earth, and sometimes farther away. The time when it is nearest is called “perigee” and the time when it is farthest is called “apogee.”

What we are most interested in is perigee, the date and time when the moon is closest to Earth. This month perigee occurs on Sunday, August 30, at 11 a.m. EDT, about 18 hours after full moon. At that time, the moon will be 222,631 miles (358,290 km) away from Earth.

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The moon will be closest to the Earth, called “perigee,” the next morning at 11 a.m. Credit: Starry Night software.

Notice how less than a day’s change in position makes it clear that the moon is no longer full: you can see that it is lit more from the left side.

Events like full moon and perigee occur at exactly the same time when viewed from anywhere on Earth, even though the local time on the clock may look different. This month both full moon and perigee occur when it is daytime in North America, and the moon is below the horizon. The best time to see the full moon at perigee will be on Saturday evening, August 29. The moon will be a few hours past full, and perigee will be a few hours in the future, but that’s the closest we can get this month.

The important thing for astronomers is that the perigee distance is less than 360,000 kilometers. When the moon gets this close, its most important effect on the Earth, the ocean tides, gets stronger. On the day of perigee and the three days following, we will have larger tides than usual.

Looking ahead to next month, full moon will fall on Sunday, September 27 at 10:51 p.m. EDT, and perigee just 51 minutes earlier at 10 p.m. This perigee will be the closest in 2015, 221,753 miles (356,877 km). The result will be the largest full moon of the year, and even larger high tides. Notice that both events happen in the evening when the moon will be well placed in the sky.

The full moon of September is traditionally called the harvest moon, because it rises around sunset on several successive nights, giving farmers extra light in the evening to bring in their harvest.

The September full moon will also pass through the Earth’s shadow, a total eclipse of the moon, visible on the evening of September 27 in North and South America, and the morning of September 28 in Europe and Africa.

When the moon is close to full, we can see the strong contrast between its grey plains and white mountainous regions, which some people see as the “man in the moon” and others see as a rabbit.

With the naked eye or a small binocular, you can easily see the three main dark plains on the upper half of the moon, which bear the fanciful names of the Oceanus Procellarum (Ocean of Storms), Mare Serenitatis (Sea of Serenity), and Mare Imbrium (Sea of Rains). These were named before we knew that there were no open bodies of water on the moon, and no atmosphere to cause storms or rains. Look also for the brightest crater on the moon, Tycho, with its beautiful system of rays, cause by material expelled when it was caused by the impact of an asteroid millions of years ago.

Blue Moon Rises On Friday

On Friday, much of the world will have a “blue moon.”

The moon won’t actually be blue in color. The expression “once in a blue moon” is a statement about the rarity of an event, not about the moon’s color.

The expression “once in a blue moon” existed long before anyone tried to define exactly what a “blue moon” is. I believe that someone tried to come up with something about the moon that rarely happens, and got the idea that it might be an “extra” full moon in a given period of time.

In most years we have 12 full moons, one in each month. But because the lunar month is slightly shorter than the calendar month, some years, such as 2015, have 13 full moons. The synodic month (full moon to full moon) averages 29.530589 days, which is shorter than every calendar month in the year except February. Those extra 1/2 or 1-1/2 days accumulate over the year, causing the date of full moon to migrate through the calendar month.

To see what I mean, here is a list of full moon dates in 2015:

January 5
February 3
March 5
April 4
May 4
June 2
July 2
July 31
August 29
September 28
October 27
November 25
December 25

If we look forward to 2016, we see the date of full moon continuing to move forward through the month until it falls on the December 14 at the end of the year.

Full moon is an instantaneous event, when sun, Earth and moon fall close to a straight line. It occurs at the same instant everywhere in the world, whether the moon is above or below the horizon. The full moon on July 31 occurs at exactly 10:43 Coordinated Universal Time (UTC), the time that astronomers and pilots use. It is equivalent to Greenwich Mean Time (GMT), the local time in Greenwich, the suburb of London England where the Royal Observatory was established by King Charles II in 1675.

 Universal Time always uses the 24-hour clock, no “a.m.” or “p.m.” The day starts at midnight (0h), noon is 12h, and the last minute of the day begins at 23h 59m.

The time zones across the world are all measured from Greenwich and expressed in the number of hours they differ from Greenwich time. In most of eastern North America, we are currently on Eastern Daylight Time (EDT), which is obtained by subtracting 4 hours from GMT or UTC, so full moon occurs at 6:43 a.m. EDT.

As we move west across North America, we have to subtract more hours from UTC to get the local time. For example, on the west coast, we use Pacific Daylight Time, which subtracts 7 hours from UTC. Full moon occurs at 3:43 a.m. PDT.

So, where did the idea come from that this extra full moon should be called a “blue moon”? The idea seems to have originated in the Maine Farmers' Almanac, which started listing the quarter of the year in which 4 rather than 3 full moons occurred. In 1946, Sky & Telescope published an article which misinterpreted the older definition and defined a blue moon as the second full moon in a month. This definition was forgotten until the popular board game Trivial Pursuit used it as one of its trivia questions in 1986. This made this rather arcane definition mainstream.

So, when you look at the “blue moon” on Friday morning, don’t expect to see any different color. Just be aware that it is a byproduct of the many compromise we make so that our calendar works well most of the time.