Keeble Observatory
May 2007 Sky from the Keeble Observatory
We’ve written about this before, but sometimes things bear
repeating! A “Blue Moon” once meant something impossible,
or at least highly unlikely, much like “when donkeys fly!”
This was apparently the usage of this term as early as the 16th
Century.
In 1883, the explosion of Krakatau in Indonesia threw enough dust
into the atmosphere to turn world-wide sunsets green, and the Moon
blue – forest fires, prolonged drought, and volcanic eruptions
can still do this. So the Blue Moon became synonymous with something
rare, hence “once in a Blue Moon.”
The connection of a Blue Moon with the calendar apparently comes
from the Maine Farmers’ Almanac in 1937. That Almanac relies
on the “tropical year,” which runs from Winter Solstice
to Winter Solstice. The seasons are not identical in length, since
Earth’s orbit is elliptical rather than circular. Further,
the synodic month is approximately 29.5 days, which doesn’t
fit evenly into a 365.24-day tropical year, nor into seasons only
approximately three months in length. Most tropical years have 12
Full Moons, but occasionally there will be 13, so one of the seasons
will get four. They called a “Blue Moon” the occasional
third Full Moon in that season in which there happened to be four.
(The Full Moons closest to the equinoxes and solstices already have
traditional names.) Their version was misinterpreted by J. Hugh
Pruett, writing in Sky and Telescope in 1946, to mean the second
Full Moon in a given month. That version was repeated in a broadcast
on National Public Radio’s Star Date in 1980, and the definition
has stuck!
While it is true that the phrase comes from a folk tale, the current
folk tale isn’t very old! So, when someone talks about a Blue
Moon today, they are referring to the second Full Moon in a month.
And, we’ll have one in May!
Lunar phases for May: Full Moon at 6:09 am, on the 3rd; Last Quarter
on the 10th, at 12:27 am; New Moon on the 16th, at 3:27 pm; and
First Quarter on the 23rd, at 5:03 pm. And … another Full
Moon on the 31st at 9:04 pm! Happy Blue Moon!
Mars still rises to the east, about 2 hours before the Sun reaches
the horizon, moving slowly up and toward the southeast, where it
will be seen about 20 degrees off the horizon at sunrise. By month’s
end its lead time will be more like 3 hours, and it will be a bit
higher in the sky at sunrise. Jupiter is to the southwest, trending
lower as the month advances.
Sunset and early evening are good for planet watchers. Venus, still
brilliant to the west, will dance a pas de deux with the young crescent
Moon from the 18th through the 20th. The two stars above Venus are
Castor and Pollux, in the constellation Gemini. Saturn begins the
evening very high (about 70 degrees) above the southern horizon.
Mercury returns to the evening sky, and by the end of May will join
a line stretching from Saturn through Venus and Mercury to the Sun’s
setting point. Look for the three planets approximately equally
spaced.
Two hours after sunset at midmonth, looking towards zenith you
will see … not much! The Milky Way circles the sky at the
horizon, and zenith is the direction out of the plane of the Galaxy
towards the imaginary spot on the sky called the galactic north
pole. Ursa Major is high to the north, with the Big Dipper’s
familiar “bowl” inverted. Follow the curve of the “handle”
of the Dipper towards the east, and find Arcturus, in Bootes, about
60 degrees above the ESE horizon. Continuing the arc to the south-southeast
will bring your vision to Spica, in Virgo, about 40 degrees above
the horizon. Continue turning to the southwest. There you will see
the familiar shape of Leo, the Lion, marked by the inverted “?”
asterism.
For your own monthly star chart, you can direct your web browser
to http://www.skymaps.com.
You will find extensive descriptions of what's worth looking for,
and you can download and print a single copy for your personal use.
Copyright 2007
George Spagna