Sky & Telescope - January 2025 - SGA5

Almanac 2025
Skygazer's
40°N
What's in the
sky tonight?
When does the Sun set, and when does
twilight end? Which planets are visible?
What time does the Moon rise?
Welcome to the Skygazer's Almanac
2025, a handy chart that answers these
and many other questions for every night
of the year. It is plotted for skywatchers
near latitude 40° north - in the United
States, the Mediterranean countries,
Japan, and much of China.
For any date, the chart tells the times
when astronomical events occur during
the night. Dates on the chart run vertically
from top to bottom. The time of
night runs horizontally, from sunset at
left to sunrise at right. Find the date you
want on the left side of the chart, and
read across toward the right to find the
times of events. Times are labeled along
the chart's top and bottom.
In exploring the chart you'll find that
its night-to-night patterns offer many
insights into the rhythms of the heavens.
The Events of a Single Night
To learn how to use the chart, consider
some of the events of one night. We'll
pick January 5, 2025.
First find " January " and " 5 " at the left
edge. This is one of the dates for which
a string of fine dots crosses the chart
horizontally. Each horizontal dotted line
represents the night from a Sunday evening
to Monday morning. The individual
dots are five minutes apart.
Every half hour (six dots), there is
a vertical dotted line to aid in reading
the hours of night at the chart's top or
bottom. On the vertical lines, one dot is
equal to one day.
A sweep of the eye shows that the
line for the night of January 5-6 crosses
SGA25W
F OR LAT I T UD ES NE AR 40° NORT H
many slanting event lines. Each event line
tells when something happens.
The dotted line for January 5-6 begins
at the heavy black curve at left, which
represents the time of sunset. Reading
up to the top of the chart, we find that
sunset on January 5th occurs at 4:49 p.m.
Local Mean Time. (All times on the chart
are Local Mean Time, which can differ
from your clock time. More on this later.)
Following the dotted line for the
5th rightward, we see that at 5:38 p.m.
the red planet Mars rises, a sign it will
become well placed for viewing later on.
The dashed line at 6:26 p.m. tells when
evening twilight technically ends. This
is when the Sun is 18° below the horizon
and the sky becomes fully dark.
At 8:01 Polaris, the North Star, has
its upper culmination. It then stands
directly above the north celestial pole
(by 38′ or 37′ this year), a good time to
check the polar alignment of an equatorial
telescope mount.
Venus, which has been the brilliant
" evening star " up until now, finally sets
in the west at 8:42.
At 8:44 the Pleiades transit the meridian,
meaning the famous star cluster is
due south and highest in the sky. So we
know it will be a fine target to enjoy in
binoculars or a telescope all evening. The
Great Orion Nebula (Messier 42) transits
at 10:33, as does Sirius at 11:42. Transit
times of such celestial landmarks help us
keep track of the march of constellations
across the night sky.
Notice the tiny Moon symbol sitting
on the January 5-6 dotted line at about
11:26. You can tell from the legend at the
bottom of the chart that it is at waxing
crescent phase, setting.
Running vertically down the midnight
line is a scale of hours. This shows
the sidereal time (the right ascension of
objects on the meridian) at midnight. On
January 5-6 this is 7h 04m. To find the
sidereal time at any other time and date
on the chart, locate that point and draw a
line through it parallel to the white event
lines of stars. See where your line intersects
the sidereal-time scale at midnight.
(A star's event line enters the top of the
chart at the same time of night it leaves
the bottom. Sometimes one of these segments
is left out to avoid crowding.)
Near the midnight line is a white
curve labeled Equation of time weaving
narrowly right and left down the chart.
If you regard the midnight line as noon
for a moment, this curve shows when
the Sun crosses the meridian and is due
south. On January 5th the Sun runs slow,
transiting at 12:06 p.m. This deviation,
important for reading a sundial, is caused
by the tilt of the Earth's axis and the
ellipticity of its orbit.
At 1:08 a.m. Mars finally reaches the
meridian, making this the very best time
of night to look for surface markings in a
good telescope. The wee hours continue,
and at 5:02 Jupiter sets just as Antares,
a star we normally associate with a later
season, rises.
The first hint of dawn - start of
morning twilight - comes at 5:45. The
Sun finally peeks above the horizon at
7:22 a.m. on January 6th.
Other Charted Information
Many of the year's chief astronomical
events are listed in the chart's evening
and morning margins. Some are marked
on the chart itself.
Conjunctions (close pairings) of two
planets are indicated by a symbol on
the planets' event lines. Here, conjunctions
are considered to occur when the
planets actually appear closest in the sky,
not merely when they share the same
ecliptic longitude or right ascension.
Opposition of a planet, the date when
it is opposite the Sun in the sky and thus
visible all night, occurs roughly when its
Sky & Telescope - January 2025

Table of Contents for the Digital Edition of Sky & Telescope - January 2025
