and Sunset Times for
The length of daylight has increased a few minutes
since the winter solstice, but the time of sunrise
does not change during the first eleven days of
January. Since the declination (angle) of the sun
above the horizon determines the amount of daylight,
sunrise should occur earlier after the winter solstice
just as sunset occurs later. However, the Earth’s
distance from the sun influences the time of sunrise.
On January 2, Earth reaches perihelion, its closest
point to the sun. If the Earth’s orbit were
a perfect circle, a solar day (not always 24 hours
like a clock day) would equal one rotation of the
Earth’s axis. Since the orbit is an ellipse,
the sun’s gravitational force causes the
Earth to move faster as it approaches perihelion.
The result is that the sun is not directly overhead
after a rotation of 360 degrees. Instead, the sun
is east of the meridian (point directly overhead)
because the Earth has moved farther along in its
orbit which changes its position relative to the
sun. Between November and February each solar day
is longer because of the extra time required for
the sun to reach the meridian. Noon arrives a few
seconds later each day. A late noon delays sunset.
The effect was noticed when the sun began to set
later after December 13, before the winter solstice.
Delayed sunsets also delay the following sunrise
which explains why the sun is not rising earlier
during the first few weeks of January. By January
12 the declination of the sun is high enough in
the sky to overcome the effect of a longer solar
January begins with a waning moon high in the morning
sky at sunrise. Just before dawn on January 4 look
for a crescent moon forming an isosceles triangle
with Venus and Antares, a 1st magnitude star in Scorpius.
New Moon occurs on January 8 at 4:37am. On January
17 the moon grazes the star cluster Pleiades just
before midnight. On January 19 a waxing gibbous moon
hovers above Mars. A nearly full moon rises on January
21 at 4:36 pm. Full Moon occurs in the morning twilight
of January 22 at 6:35am. On January 24 the moon and
Saturn both rise shortly after 8:00pm with Regulus,
a 1st magnitude star in Leo just below. Best viewing
will be soon after 9:00pm.
MAGNITUDE OF THE STARS
Astronomers use the term magnitude to measure
the brightness of stars. Absolute magnitude measures
actual differences in size and intensity of a star’s
light if all stars were the same distance from Earth.
Apparent magnitude, more useful to stargazers, ranks
a star’s brightness based on what we actually
see. A small star near Earth appears brighter than
a large more distant star. The original magnitude
scale had six ranks. The brightest stars were ranked
1st magnitude. Magnitude 1 stars are 2.5 times brighter
than magnitude 2 stars; magnitude 2 stars 2.5 times
brighter than magnitude 3 stars; etc. A magnitude
1 star is 100 times brighter than a magnitude 6 star,
which is the faintest level that can be viewed without
magnification. Well-marked constellations, like the
Big Dipper, Cassiopeia, and Orion, are defined by
1st and 2nd magnitude stars. Fainter constellations,
like Pisces and Cancer, contain only 3rd or 4th magnitude
stars. The current magnitude scale has been extended
to -26.8 to accommodate the brightness of the sun
and to +27 to accommodate the faintest objects seen
with the largest telescopes. Some of the brighter
stars like Rigel, Capella, and Vega are now designated
magnitude 0. Sirius, the brightest of all stars,
is -1.6; Venus ranges from -3.9 to -4.7.
Look low in the northeastern skies for meteors from
the Quadrantid Meteor Shower during the first week
of January. The Quandrantids, renown for their abundant
though faint meteors, peak on the night of January
3 between midnight and early twilight on the morning
of January 4.
Be sure to check the progress of Comet 17P/Holmes
as it passes through Perseus during December. Its
outburst of dust and ice on October 24, 2007, increased
its brightness by a million times in just a few days.
By mid-November the diameter of the dust and ice
cloud forming its coma was larger than the diameter
of the sun. Comet Holmes went through a similar outburst
and increase in brightness in 1892 when it was first
discovered. Normally it is no brighter than the planet
Pluto. The comet is fading in brilliance as it expands,
but it could repeat its performance of 1892 when
the first outburst was followed by another a few
months later. Comet Holmes has an orbital period
of seven years. Another Comet, 8P/Tuttle, is due
to rendezvous with the sun around the beginning of
the new year. Look for it as it passes through Cassiopeia
and Andromeda during December. Its brilliance is
similar to that of a dim star when viewed with the
unaided eye. Comet Tuttle, with a period of 13.6
years, is the source of the Ursid Meteor Shower.
Jupiter - returns mid-January
shining at magnitude -1.9 below Venus in the morning
sky; on January 31 both Venus and Jupiter rise
within a few minutes of 5:30am.
Mars - centered amongst four
of the brightest stars in the night sky—Capella
to the north, Betelgeuse to the south, Pollux
to the east, and Aldebaran to the west; its reddish
light shines brighter (magnitude -1.5) than any
star in the night sky other than Sirius (bluish
star to the southeast at magnitude -1.6); by
month’s end, Mars loses half its apparent
brilliance as it moves farther from Earth.
Mercury - visible in the southwestern
sky from a high plateau about 40 minutes after
sunset January 12-28; on the 9th look for it
near the crescent moon.
Saturn - rises mid-evening at
magnitude 0.7 with the constellation Leo; still
visible in the morning sky to the west.
Venus - bright morning star
at magnitude -4; rises around 4:30am on January
1, then about one minute later each day, rising
around 5:30am by the end of the month; Venus
can sometimes be spotted during the day; note
its location relative to the sun at sunrise,
then look for a white spot in the blue sky to
the west of the sun later in the day.
Uranus - shining at magnitude
5.9, its blue-green tint is barely perceptible
to the unaided eye in dark sky; look for it in
Comet Holmes remains in Perseus throughout January.
Its brilliance has diminished as its coma (dust and
ice surrounding its solid core) expands. Comet Tuttle
makes its closest approach to Earth on January 2. Look
overhead between Aries and Pisces. Tuttle, as dim as
a faint star, moves quickly through the night sky because
it is so near the Earth, reaching the southern horizon
by the end of January. Use binoculars for best viewing
of both comets.