A supermoon will delight skygazers on the night of 8 September. This event coincides with the Harvest Moon.
Supermoons can result in both the apparent size, as well as the brightness, of our natural satellite being greater than normal. They occur because the changing distance between the Moon and our home planet. At its closest, or perigee, our lone natural satellite comes within 225,623 miles of the Earth. The greatest point of separation in its orbit, known as apogee, lies 252,088 miles away from our world.
Observers on the ground witness a full moon when the moon lines up in a line with the Sun and Earth, with out planet in the middle. These are the times when "spring tides" swell water to the greatest degree. The term refers to ocean waters "springing up," and has nothing to do with the season.
When the Moon reaches full phase at the same time as perigee occurs, the event is known to the public as a supermoon. Astronomers prefer the term perigee moon.
Three supermoons occurred in 2014, on 12 July, 10 August, and 9 September. During the August event, the moments of greatest illumination and perigee were just one hour hour. That coincidence made this second event the most dramatic of the three occurrences.
"Generally speaking, full Moons occur near perigee every 13 months and 18 days, so it's not all that unusual. In fact, just last year there were three perigee Moons in a row, but only one was widely reported," Geoff Chester of the US Naval Observatory said.
Brightness and apparent size of the Moon are difficult to measure with the unaided eye. Ice crystals high in the atmosphere, dust, pollution, and local light pollution can alter perception of what is seen. The Moon will often look larger and brighter than normal when it is seen near the horizon, due to a psychological effect called the Moon illusion. The satellite appears larger than normal when seen behind a backdrop of houses, trees, and other familiar objects.
The Moon is slowly moving away from the Earth at a rate of about 1.5 inches each year. This is caused by tidal forces constantly stretching and warping the surfaces of both bodies.
Harvest moons are defined as the full moon closest to the September equinox, when many crops are ready to bring in from the fields. This year, this full moon occurs at the beginning of the month, nearly the earliest possible time in the year it can take place.
No special equipment is needed to see the supermoon, although dark skies, away from city lights, offer the best viewing.
