Photo of the Week.The top of the 22-degree halo
around the Sun caused by refraction of sunlight through hexagonal
ice crystals.
Astronomy news for the week starting Friday, January 9, 2015.
This week centers on the third quarter
Moon, which takes place on Tuesday, January 13, shortly before
daybreak in North America, allowing a near-perfect quarter phase
to be seen. Look for the star Spica in Virgo just to the south. In the third century BC,
Aristarchus of Samos tried to measure the angle between the Sun and
Moon at the time of the quarter. If the Sun were infinitely far
away, the angle would be 90 degrees. He found an angle of 87
degrees, and concluded that the Sun was 20 times farther than the
Moon. It turns out that the angle is impossibly small to find
with the naked eye and he was off by a factor of 20 (the Sun 400
times farther than the Moon). But his brilliant conclusion that
the Sun is much more distant than the Moon was indeed correct.
Earlier in the week we see the Moon in the waning gibbous phase, later in the week it
thins as a waning crescent. The morning
of Friday the 16th, as Skylights week ends, the rising crescent
glides just to the north of Saturn, the two
of them above the star Antares
in Scorpius.
As the week opens, the Moon is at apogee
, where it is farthest from Earth. And because of the
eccentricity of the Earth's orbit and the tilt of the terrestrial
rotation axis, the latest sunrise took place around January 4, the
latest dawn around the 8th. We will now see the morning skies
ever so slowly getting lighter.
The evening sky features Venus, which
is slowly breaking out of dusk as it climbs higher and higher.
This week features a close interplay between Venus and Mercury, which passes
greatest eastern elongation (to the east of the Sun) on Wednesday
the 14th. The evening of Saturday the 10th, the two will be under
a degree apart (Mercury the fainter), allowing an excellent
opportunity to see the first planet from the Sun, providing you
have an unobstructed west-southwestern horizon. Shortly after
Venus sets and evening twilight dims, Jupiter rises
in the east still to the west of Regulus in Leo. Climbing high, the giant planet crosses the meridian to the south around 2 AM. An
hour and a half later, Saturn (fainter than Jupiter, but still
bright first magnitude) rises right off the tip of Scorpius's
three-star head. Back in the evening,
Mars continues to set at 8 PM near the Capricornus-Aquarius
border. Then catch
Comet Lovejoy, which this week zooms
northward west of
Orion and
Taurus; binoculars will help.
In the early evening, the constellations of the Andromeda myth fill much of the
northern sky. The most southerly of them, Cetus, the Sea Monster or Whale, floats with his head
above celestial equator. Higher and
a bit to the right is the flat triangle that makes Aries (the Ram), next upward the
eponymous Triangulum, and then the
focus of the story, Andromeda herself.
