The last time the moon was full on New Year’s Day occurred in 1999. After Monday, the next time it happens is 2037. It turns out that every 19 years a full moon occurs on Jan. 1, and a second full moon (sometimes known as a “blue moon”) occurs on Jan. 31. This last happened in 1999, it happens again Monday and will happen after that in 2037.

You can choose any date of the year, and the phase of the moon on that date repeats every 19 years.

The Hebrew calendar, which is based on the moon, observes that the sun, Earth, and moon come back into the same relative positions every 19 years. This relationship is actually known as the Metonic cycle. Meton of Athens introduced the concept in 432 B.C., though it probably was known by Neolithic peoples more than 2,000 years earlier. This represents the complex relationship of the Earth’s rotation around the sun every 365.25 days and the length of the lunar month of 29.5 days. Moreover, these relationships are superimposed on our modern Gregorian calendar, which defines a year as 365 days plus a leap year day every four years.

We have modern technology able to measure time in microseconds and predict celestial events quite accurately, but the ancients were just as clever and much better observers of the sky.

The last full moon was on Dec. 3 and was also the only supermoon to take place in 2017. A “supermoon” occurs if the moon is at its closest approach to the Earth when it is full.

The average distance between the center of the Earth and the center of the moon is 239,228 miles. This distance varies from a low of 221,500 miles at perigee (moon closest to the Earth) to a high of 252,700 miles at apogee (moon farthest from the Earth).

This Sunday the moon will be about 222,272 miles from the Earth.

A supermoon is not all that super, but the moon may appear a bit larger and brighter.

Supermoons are more of an astrological term than an astronomical term. Nevertheless, they have caught the popular fancy.

One photo shows the moon in March 2016 and November 2016, when the moon was, respectively, at its farthest distance from the Earth (apogee) when full and when the moon was at a particularly close approach (perigee) to the Earth when full.

The second picture shows a project Hunter recently completed of imaging the full moon every month during 2017 and putting the same scaled images side by side so one can see the relative change in the moon’s orientation and apparent size from month to month.

He labeled the dates and the distances for the full moon on each month when the images were taken. One can see there is a definite but somewhat subtle difference in size of the full moon from month to month depending on its distance from the Earth. For an experienced moon observer, a supermoon will seem a bit larger and brighter than an ordinary full moon, but, at most, the moon is only about 14 percent larger in diameter when it is full at perigee as compared to apogee.

The best place to watch a supermoon is your own backyard or the park near your house.

All you need is a clear sky and the moon not hidden by trees or a nearby building.

Catch the moment

The moon is most amenable to being photographed. It is relatively large and bright. With an ordinary cell phone camera, it will be a small dot, but you can spice up the picture by trying to zoom the picture somewhat and putting the moon next to a building, interesting tree, or church steeple for a more dramatic scene.

This works much better with a digital camera having a telephoto lens and the camera stabilized by a tripod. Telephoto lenses with focal lengths 300 mm and greater will show the moon with reasonable detail and offer the opportunity to silhouette a church steeple, cell tower, tall tree, or other structure on the moon. You will have to experiment with exposures and f/ratio settings. There are endless possibilities of photographing a supermoon as part of a landscape photograph.

Contact Tim Hunter at skyspy@tucson.com