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.