Sky & Telescope - January 2021 - SA8
What's in the
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
2021, 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 10, 2021.
First ﬁnd "January" and "10" 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 10-11 crosses
L AT I T U DE S
many slanting event lines. Each event line
tells when something happens.
The dotted line for January 10-11
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 10th occurs
at 4:54 p.m. Local Mean Time. (All times
on the chart are Local Mean Time, which
can differ from your clock time. More on
We see that Saturn, Mercury, and
Jupiter all set in quick succession around
6 p.m. This means they are fairly close
together in the sky, as described in the
column of text left of the chart. But they
may be hard to see so soon after sunset.
At 6:18 p.m. the bright star Sirius
rises. Then at 6:31 a dashed line shows
that twilight technically ends. This is
when the Sun is 18° below the horizon
and the sky is fully dark.
The red planet Mars transits the
meridian at 6:34, meaning it is due south
and "riding high," an excellent time to
study it in a telescope.
At 7:36 Polaris, the North Star, has
its upper culmination. It then stands
directly above the north celestial pole (by
39′ or 38′ this year), a good time to check
the alignment of an equatorial telescope.
The famous Pleiades star cluster transits at 8:25 p.m., followed by the Orion
Nebula (10:13) and Sirius (11:22). Transit
times of such celestial landmarks help us
keep track of the march of constellations
across the night sky.
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 10-11 this is 7h 24m. 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.
N E A R
(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 10th the Sun runs
slow, transiting at 12:08 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.
After being visible all evening, Mars
ﬁnally sets at 1:22 a.m. The wee hours
continue, and at 4:41 Antares rises. This
is a star we usually associate with a much
later season of the year.
The ﬁrst hint of dawn - start of
morning twilight - comes at 5:45 a.m.
A beautiful crescent Moon rises around
6:01, as does Venus at 6:12. The Sun
ﬁnally peeks above the horizon at 7:21
a.m. on January 11th.
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
the planets' event lines. Here, conjunctions are considered to occur when the
planets actually appear closest in the sky
(at appulse), not merely when they share
the same ecliptic longitude or right
Opposition of a planet, the date when
it is opposite the Sun in the sky and thus
visible all night, occurs roughly when its
transit line crosses the Equation-of-time
line (not the line for midnight). Opposition is marked there by a
for Saturn on the night of August 1-2.
All events on this Skygazer's Almanac
are plotted for an observer at 90°° west
longitude and 40° north latitude, near
the population center of North America.
However, you need not live near Peoria,
Illinois, to use the chart. Simple corrections will allow you to get times accurate to a couple of minutes anywhere in
the world's north temperate latitudes.
Rising or Setting Corrections
Declination (North or South)
5° 10° 15° 20° 25°
Moonrise and moonset can be told
apart by whether the round limb - the
outside edge - of the Moon symbol faces
right (waxing Moon sets) or left (waning Moon rises). Or follow the nearly
horizontal row of daily Moon symbols
across the chart to ﬁnd the word Rise or
Set. Quarter Moons are indicated by a
larger symbol. Full Moon is always a large
bright disk whether rising or setting; the
circle for new Moon is open. P and A
mark dates when the Moon is at perigee
and apogee (nearest and farthest from
Mercury and Venus never stray far from
the twilight bands. Their dates of greatest
elongation from the Sun are shown by ◗
symbols on their rising or setting curves.
Asterisks mark their dates of greatest
illuminated extent in square arcseconds.
For example, this occurs for Venus on the
evening of December 3rd this year.
Meteor showers are marked by a starburst symbol on the date of peak activity
and at the time when the shower's radiant is highest in the night sky. This is
often just as morning twilight begins.
(Note that we've moved the peak of the
Southern Taurids, a sparse, ill-deﬁned
shower, to a little earlier in the year.)
Julian dates can be found from the
numbers just after the month names on
the chart's left. The Julian day, a sevendigit number, is a running count of days
beginning with January 1, 4713 BC. Its
ﬁrst four digits this year are 2459, as
indicated just off the chart's upper left
margin. To ﬁnd the last three digits for
evenings in January, add 215 to the date.
For instance, on the evening of January 10th we have 215 + 10 = 225, so the
Julian day is 2,459,225. For North American observers this number applies all
night, because the next Julian day always
begins at 12:00 Universal Time (6:00
a.m. Central Standard Time).
To convert the charted time of an
event to your civil (clock) time, the following corrections must be made. They
are mentioned in order of decreasing
* DAYLIGHT-SAVING TIME. When this
is in effect, add one hour to any time
obtained from the chart.
* YOUR LONGITUDE. The chart gives the
Local Mean Time (LMT) of events, which
differs from ordinary clock time by a
number of minutes at most locations.
Our civil time zones are standardized
on particular longitudes. Examples in
North America are Eastern Time, 75°
W; Central, 90°; Mountain, 105°; and
Paciﬁc, 120°. If your longitude is very
close to one of these (as is true for New
Local Mean Time Corrections
Salt Lake City +28
San Francisco +10
Orleans and Denver), luck is with you
and this correction is zero. Otherwise, to
get standard time add 4 minutes to times
obtained from the chart for each degree
of longitude that you are west of your
time-zone meridian. Or subtract 4 minutes for each degree you are east of it.
For instance, Washington, DC (longitude 77°), is 2° west of the Eastern Time
meridian. So at Washington, add 8 minutes to any time obtained from the chart.
The result is Eastern Standard Time.
Find your time adjustment and memorize it. The table below left shows the
corrections from local to standard time,
in minutes, for some major cities.
* RISING AND SETTING. These times need
correction if your latitude differs from
40° north. This effect depends strongly
on a star or planet's declination. (The
declinations of the Sun and planets are
listed monthly in Sky & Telescope.)
If your site is north of latitude 40°,
then an object with a north declination stays above the horizon longer than
the chart shows (it rises earlier and sets
later), whereas one with a south declination spends less time above the horizon.
At a site south of 40°, the effect is just
the reverse. Keeping these rules in mind,
you can gauge the approximate number
of minutes by which to correct a rising or
setting time from the table above.
Finally, the Moon's rapid orbital
motion affects lunar rising and setting
times if your longitude differs from 90°
west. The Moon rises and sets about two
minutes earlier than the chart shows
for each time zone east of Central Time,
and two minutes later for each time zone
west of it. European observers can simply
shift each rising or setting Moon symbol
leftward a quarter of the way toward the
one for the previous night.
For reprints (item SGA21W, $4.95 each) or to order
a similar chart for latitude 50° north or 30° south,
go to: shopatsky.com/resource-materials/calendarsalmanacs
Skygazer's Almanac 2021 is a
supplement to Sky & Telescope
Magazine, One Alewife Center,
Suite 300B, Cambridge, MA
02140, USA, skyandtelescope.
org. ©2020 AAS Sky Publishing, LLC. All rights reserved.
Sky & Telescope - January 2021
Table of Contents for the Digital Edition of Sky & Telescope - January 2021
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