18th
The reason behind the big bright moon
Once in a Close Moon
Tonight will be the brightest and closest full moon since 1993.
Tonight is the perigee - the closest the Moon gets to Earth during its ecliptic orbit. As a result the Moon will be 30% brighter and 14% larger. The time when the Moon is its furthest away from Earth is the apogee.
These events usually occur once a month, but due to the irregular orbit the exact distance varies over the year. Also, the moon’s phase differs.
Ed Krupp, director of the Griffith Observatory in Los Angeles said: “Typically we don’t have the full moon phase and perigee coinciding at the same time, so that makes this event particularly special.”
At a distance of 221,560 miles (356,566 lm), tonight will be the closest lunar perigee since 1993. Quite interestingly, the Moon’s farthest apogee this year will be on Boxing Day, when the Moon will be 252,650 miles (406,601 km) away from Earth.
The unusual closeness of tonight’s Moon will result in higher-than-average tides, but measuring the other effects of the perigee is harder to discover. Most casual observers will only notice the Moon being brighter rather than being larger.
Krupp added that one of the best things about this event is that anyone in the world can watch it without the need for special equipment. All that is needed is clear skies. He said: “If you are charmed by the idea of seeing the biggest and brightest full moon visible in 15 years, be ready to go outside at sunset and watch for the rising Moon in the east. Or stay up all night and watch as the Moon rides through the overhead skies—either way it will be a beautiful sight.”
Story from National Geographic via the QI website
