For those that don’t remember this ~monthly feature, I get an email in the inbox once every four weeks from Gordon Johnston. For those that don’t know Gordon, is the Program Executive (think NASA HQ’s head engineer) for the Lunar Reconnaissance Orbiter and for OSIRIS-REx. He also happens to be a pleasant office-neighbor (he “lives” on the other side of the one wall my cubicle has). Anyways, Gordon knows his moon. And he sends an email every lunar cycle with viewing opportunities and historical information for the next four weeks of space-gazing. I pass it on to you in full with his permisison.
The next full Moon will be on Saturday evening, May 5, 2012, reaching its
peak at about 11:36 pm EDT. The Moon will appear full for about 3 days
around this time, from Friday evening through Monday morning. As usual,
copacetic celebratory celestial costuming is should be considered in support
of our sated Selene.
This is the Flower Moon, as in most areas flowers are abundant this time of
year. Other names include the Corn Planting Moon or the Milk Moon.
This will be the brightest full Moon of 2012 (sometimes called a “Super
Moon”). The Moon reaches its closest point to the Earth in its orbit
(called perigee) at 11:28 pm EDT, just about the same time as the Moon is
closest to opposite the Sun for the month. Last year the full Moon March
was the biggest and brightest full Moon of 2011. In 2013 it will be in
June. We are coming out of a period when the Moon is brighter than usual in
winter and moving towards a time when the Moon is brighter than usual in the
What makes some full Moons brighter than others? There are a number of
factors, some predictable far in advance, others not so predictable.
– Of course, the weather makes the biggest difference and can’t really be
predicted more than a week or so in advance – the Moon will not appear
bright if you can’t see it because of clouds, etc.
– For a similar reason, the Moon appears brighter when it is high in the
sky and shining through less atmosphere (especially if the atmosphere is at
all dusty, hazy, smoggy, or cloudy). Because the full Moon is always close
to opposite the Sun the full Moon closest to the Winter Solstice rises
highest in the night sky.
– Also, when the Moon is high in the sky, you are closer to the Moon by
about the radius of the Earth compared to when the Moon is on the horizon.
Since the distance from the center of the Earth to the center of the Moon is
on average about 384,403 km, the radius of the Earth about 6,371 km, and
brightness changes as the square of the distance, being closer to the Moon
by about the radius of the Earth increases the brightness of the full Moon
by about 3%.
– It also makes a difference how much sunlight is getting to the Moon. The
orbit of the Earth-Moon system around the Sun is not a perfect circle, and
since the intensity of sunlight changes as the square distance, this
difference accounts for about a 6.7% variation in the brightness of the
sunlight reflecting off the Moon towards the Earth. The Earth-Moon system
is at its closest to the Sun (called perihelion) about 14 days after the
Winter Solstice for the Northern hemisphere, in early January.
– Just like the retro-reflectors on a bicycle or a car, the Moon reflects
more sunlight directly back towards the Sun. If you were on a spacecraft
looking at the Moon, you might see as much as a 40% increase in the
brightness in the sunlight near your own shadow (as opposed to looking 4
degrees away from your shadow [Burrati, B. J.; Hillier, J. K.; & Wang, M.
(1996) “The Lunar Opposition Surge: Observations by Clementine”. Icarus 124:
490-499]). However, if the Moon is too close to opposite the Sun, it falls
into the Earth’s shadow, so this limits how much of the “opposition effect”
we see from the Earth. I don’t have a good estimate of how much variation
the opposition effect causes from one full Moon to another.
– The biggest predictable effect on the brightness of the full Moon is how
close the Moon is to the Earth. With everything else the same, a full Moon
is about 30% brighter when the Moon is closest to the Earth in its orbit
(called perigee) compared to a full Moon when the Moon is farthest from the
Earth in its orbit (called apogee).
As for other celestial events between now and the full Moon after next:
As May begins, for the Washington, DC area, the morning sky starts getting
bright enough to allow sailors at sea to detect the horizon (called nautical
twilight) a little after 5 am, with sunrise a little after 6 am EDT. Sunset
is around 8 pm EDT with nautical twilight ending a little after 9 pm. When
the sky gets dark in the evening, Venus is bright in the west northwest,
Mars is riding high almost directly overhead, while Saturn is rising from
the east southeast. By the end of the month Venus will be moving too near
the Sun to see (we will actually have a very rare Venus transit in front of
the Sun on June 5, 2012), Mars will have shifter more towards the west,
while Saturn will be riding higher in the southeastern sky.
On Thursday, May 3 and Friday, May 4, 2012, the nearly full Moon will appear
high in the sky near the bright planet Saturn and the bright (but not as
bright as Saturn) star Spica.
The Eta Aquarid meteor shower will peak before dawn on Friday, May 4, 2012.
Unfortunately, the nearly full Moon will interfere with viewing. In the
mid-northern latitudes (such as near Washington, DC), the Moon sets around
4:40 am EDT and the horizon begins to brighten with dawn around 5 am EDT.
This meteor shower is dust from Halley’s comet. This shower has a fairly
broad peak, so you may be able to see meteors the mornings before the peak
(after the peak, the nearly full Moon will remain in the sky through dawn).
On Saturday, May 19, 2012, the Moon will be at its farther point to the
Earth in its orbit (called apogee). Because this is close to the New Moon,
we will not be able to see the Moon at apogee.
On Sunday, May 20, 2012, the Moon will move between the Earth and the Sun,
but because the Moon is near apogee, the Moon appears too small to fully
block the Sun, so this will be what is called an annular eclipse. The
annular eclipse will only be visible from the northern California and
southwestern Oregon, but the partial eclipse may be visible (weather
permitting) from most of western North America. The annular eclipse will
also be visible from parts of China and Japan before it sweeps across the
Pacific towards the Oregon coast.
On Tuesday, May 22, 2012, shortly after sunset, Venus and the crescent Moon
should be visible about 6 degrees apart.
On Thursday, May 31, the waxing (increasing towards full) gibbous (more than
half full) Moon will appear about 3 degrees below the bright star Spica,
with the bright planet Saturn appearing about 5 degrees above Spica.
The full Moon after next will be on Monday, June 4, 2012.