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 (1100 GMT), the moon will arrive at
perigee — the point in 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.
While the exact time of the full moon theoretically lasts just a
moment, that moment is imperceptible to casual observers. The moon
will appear full a couple of days before and after the actual full
moon most will speak of seeing the nearly full moon as "full": the
shaded strip is so narrow, and changing in apparent width so slowly,
that it is hard for the naked eye to tell in a casual glance whether
it's present or on which side it is.
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. [The Moon Revealed: 10 Surprising Facts]
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.
Any coastal storm at sea around this time will almost certainly
aggravate coastal flooding problems. Such an extreme tide is known as
a perigean spring tide, the word spring being derived from the German
springen, meaning to "spring up," and is not — as is often mistaken —
a reference to the spring season.
Spring tides occur when the moon is either at full or new phase. At
these times the moon and sun form a line with the Earth, so their
tidal effects add together (the sun exerts a little less than half the
tidal force of the moon.) "Neap tides," on the other hand, occur when
the moon is at first and last quarter and works at cross-purposes with
the sun. At these times tides are week.
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.
So a perigee moon, either rising in the east at sunset or dropping
down in the west at sunrise might seem to make the moon appear so
close that it almost appears that you could touch it. You can check
out this out for yourself by first noting the times for moonrise and
moonset for your area by going to this website of moonrise times by
the U.S. Navy Oceanography Portal.
Happy moon-gazing!
Read more: http://www.foxnews.com/
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