Blue Moon – August 20, 2013


When the moon rises Tuesday night (Aug. 20), it brings us the August full moon and in addition, it will also technically be a “Blue Moon.”

“But wait a minute,” you may ask. “Isn’t a Blue Moon defined as the second full moon that occurs during a calendar month? Tuesday’s full moon will be the only full moon of August 2013. So how can we call it a ‘Blue’ moon?”

blue-moon-august-2012-120726b-02[Blue Moon Secrets Explained (Infographic)]

Yet it still is a Blue Moon, but only if we follow a now somewhat obscure rule of astronomy. In fact, the current “two full moon in one month” rule has superseded the rule that would allow us to call Tuesday’s full moon “blue.”

The Moon will reach full phase – the third full moon of summer 2013, traditionally called the Fruit Moon in the English-speaking world. The age of the Moon is the number of days since the last new moon. Since the phases of the Moon cycle roughly once every 29 days, the Moon is full when it is around 14 days old, and is approaching new moon again by the time is approaches an age of 29 days. Usually this four-week cycle is further subdivided into four week-long sections by the times when the Moon is exactly half illuminated, at the midpoints between new and full moon, when its age is 7 days (first quarter) and 21 days (last quarter). READ MORE

Approximately speaking, the Moon is visible for all of the night when Full; that is to say, it rises at sunset and sets at sunrise. When New, the Moon is barely visible at all; it is so close to the Sun in the sky that it rises at dawn and sets at dusk. Over the course of its waxing phases – from new moon through first quarter to full moon – the Moon is visible in the evening sky and sets during the course of the night, a little over an hour later each day. Over the course of its waning phases – from full moon through last quarter to new moon – the Moon rises during the course of the night and is visible in the morning sky.

august-full-moon-airplane-outline-sid-vedulaThe Moon itself is best observed at around First or Last Quarter; at these times the Sun is illuminating a large part of the visible surface of the Moon at a very shallow angle, so that surface bumps and craters cast long shadows, providing a clear view of surface relief. At full moon, comparatively few craters are visible, though the imposing crater Tycho, the maria, and many smaller features remain readily visible.

Photographer Sid Vedula captured this amazing view of the full moon of Aug. 1, 2012, from Houston, TX, with a passing airplane in silhouette.

Many other objects in the night sky are best observed when the Moon is below the horizon. Its bright glare can prove a substantial source of light pollution when trying to search out faint Messier objects or meteor showers.

As at any time when the Moon reaches full phase, it will be brighter than at any other time of the month, and will also be visible for much of the night on account of lying almost directly opposite the Sun in the sky.

Over the nights following 20 August, the Moon will rise a little under an hour later each day so as to become prominent later in the night. Within a few days, it will only be visible in the pre-dawn and early-morning sky. By the time it reaches last quarter, around a week after full moon, it will rise at around midnight and set at around noon.

At the moment when the Moon reaches full phase, it will lie at a declination of -07°39′ in the constellation Aquarius, and so will appear high in the sky at all but the most extreme latitudes. It will be visible at all latitudes between 72°N and 87°S. Its distance from the Earth will be 365,000 km.

The circumstances of this event were computed from the DE405 ephemeris published by the Jet Propulsion Laboratory (JPL).


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