May 2009 Sky from the Keeble Observatory
In April, we introduced the cosmic microwave background (CMB) as the most
distant light we are able to detect. Recall that this represents the “echo” of the
Big Bang, light which has travelled unimpeded from a time when the Universe was
only a few hundred thousand years old. As the Universe has expanded and cooled,
so too this light has been stretched to longer wavelengths to now represent a nearly
uniform temperature of 2.725 K. (K stands for Kelvin, which is the basic unit of
temperature. Each degree on the Kelvin scale is the same as a degree on the Celsius
scale and, since it’s a named unit, we don’t precede it with the word degrees.)
The zero point of this temperature scale is minus 273.16 degrees Celsius.
Note that we said the temperature of the CMB was nearly uniform. As our measurements
become more refined, we are now able to discern small variations in this otherwise
uniform radiation field. How small? Less than one part in 10,000 – but we are able
to measure this!
The biggest variation is that the overall radiation field appears slightly hotter
in one direction and slightly cooler in the opposite direction. The hotter spectrum
is blue shifted by the Doppler effect; the cooler is red shifted. This so called
dipole anisotropy traces the overall motion of the solar system as the Sun
orbits in the Galaxy, and the Galaxy moves on its own trajectory in our Local Group
of galaxies. The net velocity is about 627 km/s (that’s 1.4 million miles per hour!)
in the direction of the constellation Hydra.
Next month we’ll finish the tale, and discuss the small variation we can discern
when we subtract the dipole anisotropy from our data.
Lunar phases for May: First Quarter on the 1st, at 4:44 pm EDT;
Full Moon on the 9th, at 12:01 am (in time zones west of here, the Full
Moon happens on the 8th!); Last Quarter on the 17th, at 3:26
am; New Moon on the 24th, at 8:11 am; and another First Quarter will
occur on the 30th, at 11:22 pm (for time zones east of here, this will
happen on the 31st!).
Planet watching fun will happen mostly for early risers (or all-nighters!) this
month. Venus and Mars rise more than an hour before sunrise, and began the month
between 17 and 21 degrees above the ESE horizon at dawn. Venus is the brighter,
Mars is reddish in hue. By the end of the month they’ll be a bit higher and toward
the east at sunrise. Mercury comes back to the predawn skies around the 18th,
so by month’s end we should see it emerging from predawn twilight to the east. Jupiter
is to the southeast at dawn, about 30 degrees above the horizon early in the month.
If you watch it through the month with binoculars or a small telescope, you may
see Neptune drifting into the field of view from the north (left). They’ll be within
a half degree by month’s end. While you’re waiting for Neptune, pay attention to
Jupiter’s moons this month, as well. Because our current line of sight to Jupiter
is almost in the plane of its equator (and the orbital plane of its moons), there
will be several occultations this month; several will be visible from central Virginia.
On the 16th, Io will be occulted by Europa, at 5:34 am. Io returns the
favor and occults Europa on the 21st, at 1:27 am. Ganymede occults Io
on the 26th, at 4:01 am. Io will occult Europa again on the 28th,
at 3:42 am.
Evening planet watchers will have to content themselves primarily with Saturn, which
starts May high to the southeast at sunset. It climbs higher through the month,
and will be about 60 degrees above the SSW horizon as it emerges from even ng twilight.
Mercury is near the Pleiades, low to the west at sunset early in the month, but
rapidly moving into the Sun’s glare by mid-month and emerging into the morning as
we move later in May.
Our overhead view at mid-month, about two hours after sunset shows little; there’s
not much near zenith. The Milky Way rings the horizon, and we are looking almost
directly out of the plane of the Galaxy. Bright stars mark familiar constellations
that ring the sky between the horizon and overhead.
To the north we see the familiar inverted “big dipper” of the constellation Ursa
Major. Turning to our right, the next bright star is Arcturus, in Bootes to the
east. Continuing our turn, we find Spica, the brightest star in Virgo to the south-southeast.
Leo is to the southwest, with its brightest star, Regulus, marking the heart of
the Lion. That’s Saturn above and left of Regulus, followed by Denebola, which marks
the tail of the Lion. Low to the west-northwest we see Castor and Pollux, low toward
the horizon in Gemini.