By now, you've probably heard of various media reports touting tomorrow's "Super Moon". Why all the hype? Is it even really a "super moon"?!
The orbit of the Moon around the Earth is not a perfect circle; it is slightly elongated meaning that there are times when the Moon is closer to the Earth than it is at other times in its orbit (see this diagram). Tomorrow's full moon just happens to coincide when the Moon reaches its closest point to Earth in its orbit (termed "perigee").
What effect does this have for tomorrow's full moon? Some calculations suggest that the Moon will appear 14% bigger and 30% brighter than when the Moon is at its furthest point from Earth ("apogee"). Moreover, apparently this rare alignment of a full moon coinciding at perigee only happens every ~19 years although the last such event was just 3 years ago. It's not clear how this 19 year period was calculated. A better question to ask is will you notice a larger moon tomorrow night compared to last month's full moon or next month's full moon. Here the answer is probably no. Still, if this event gets people outside to enjoy our night sky that is slowly disappearing because of light pollution, then I think that's "mission accomplished"! So if it is a clear night tomorrow, make sure you do take a moment to step outside, look up and enjoy this celestial treat!
When you do, the bright object to the lower left of the Moon is the ringed planet Saturn! And below Saturn, will be the bright star Spica!
NOTE: Be careful of the bad science reports you may have seen online suggesting this rare alignment triggers natural disasters. This is just poor reporting by people with poor critical thinking skills. Just for example, consider the fact that the two previous "super Moon's", in March '93 and Dec '08, passed without any incident whatsoever. This full moon will bring with it "perigean tides" but these extra high tides should be nothing to worry about according to NOAA. According to their reports, lunar gravity at perigee will cause the tides to be higher by an inch or so than normal. No big deal.