The largest full moon of 2013, a so-called "SUPERMOON," will light up the night sky this weekend, but there's more to this lunar delight than meets the eye.
On Sunday, June 23, at 7 a.m. EDT (11:00 GMT), the moon will arrive at perigee — the point in its orbit its orbit bringing it closest to Earth), a distance of 221,824 miles. Now the moon typically reaches perigee once each month (and on some occasions twice), with their respective distances to Earth varying by 3 percent.
But Sunday's lunar perigee will be the moon's closest to Earth of 2013. And 32 minutes later, the moon will officially turn full. The close timing of the moon's perigee and its full phase are what will bring about the biggest full moon of the year, a celestial event popularly defined by some as a "SUPERMOON."
You can watch a free webcast of 2013 Supermoon full moon on SPACE.com on Sunday at 9 p.m. EDT (0100 June 24), courtesy of the skywatching website Slooh Space Camera.
During Sunday's Supermoon, the moon will appear about 12.2 percent larger than it will look on Jan. 16, 2014, when it will be farthest from the Earth during its apogee.
Supermoon's big tides.
In addition, the near coincidence of Sunday's full moon with perigee will result in a dramatically large range of high and low ocean tides. The highest tides will not, however, coincide with the perigee moon but will actually lag by up to a couple of days depending on the specific coastal location.
For example, for New York City, high water (6.3 feet) at The Battery comes at 8:58 p.m. EDT on Sunday, or more than 12 hours after perigee. From Cape Fear, N.C., the highest tide (6.5 feet) will be attained at 9:06 p.m. EDT on Monday, while at Boston Harbor a peak tide height of 12.3 feet comes at 12:48 a.m. EDT on Tuesday, almost 2 days after the time of perigee.
Tidal force varies as the inverse cube of an object's distance. We have already noted that this month the moon is 12.2 percent closer at perigee than at apogee. Therefore it will exert 42 percent more tidal force at this full moon compared to the spring tides for the full moon that will coincide with apogee next January. Huge moon at moonrise.
Usually the variation of the moon's distance is not readily apparent to observers viewing the moon directly. Or is it?
When the perigee moon lies close to the horizon it can appear absolutely enormous. That is when the famous "moon illusion" combines with reality to produce a truly stunning view. For reasons not fully understood by astronomers or psychologists, a low-hanging moon looks incredibly large when hovering near to trees, buildings and other foreground objects. The fact that the moon will be much closer than usual this weekend will only serve to amplify this strange effect.