Sky & Telescope - January 2021 - SA6

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
2021, a handy chart that answers these
and many other questions for every night
of the year. This version is plotted for
skywatchers near latitude 50° north - in
the United Kingdom, northern Europe,
Canada, and Russia.
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
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2021
FOR

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 4:20 p.m. Local Mean Time. (All times
on the chart are Local Mean Time, which
can differ from your civil clock time by
many minutes. More on this later.)
Note that Mercury, Saturn, and Jupiter set in rapid succession near 5:30 p.m.,
so they must be together in the sky. But
this occurs so soon after sunset that they
will be very hard to see. Moving to the
right, we see a dashed line marking the
end of evening twilight at 6:17 p.m. This
is the time when the Sun is 18° below the
horizon and the sky is fully dark.
The red planet Mars transits the
meridian at 6:35, meaning it is due south
at its high point in the sky - a good time
to study it in a telescope. Then at 6:44
Sirius, the brightest nighttime star, rises.
At 6:56 dim Uranus transits.
Polaris, the North Star, reaches upper
culmination near 7:37. This is when
Polaris stands directly above the north
celestial pole (by 39′ or 38′ this year), a
good opportunity to check the alignment
of an equatorial telescope.
The Pleiades star cluster in Taurus
transits the meridian at 8:26, followed by
the Orion Nebula, Messier 42, at 10:14
and Sirius at 11:23. Transits of such celestial landmarks help remind us where the
constellations are during the night.
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 23m. To ﬁnd
the sidereal time at any other time and
date on the chart, locate the point for
the time and date you want, then draw a
line through it parallel to the white event
lines of stars. See where your line inter-

N E A R

5 0°

NORT H

sects 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 the previous 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.
Mars, having been up all evening,
ﬁnally sets at 1:43 a.m. The bright star
Regulus transits at 2:46. As the wee hours
continue, Antares, a star we usually associate with a later season, rises at 5:28.
The ﬁrst hint of dawn - the start of
morning twilight - comes at 5:58 a.m.
The crescent Moon comes up at around
6:27, as does brilliant Venus at 6:49. The
Sun ﬁnally peeks above the eastern horizon at 7:55 a.m. on Monday morning,
January 11th.

Local Mean Time Corrections
Amsterdam +40
Belfast
+24
Berlin
+6
Bordeaux +62
Bremen
+24
Brussels
+44
Bucharest +16
Budapest
-16
Calgary
+36
Copenhagen +10
Dublin
+25
Geneva
+35
Glasgow
+16
Halifax
+14
Hamburg
+20
Helsinki
+20
Kiev
-2
London
0
Lyons
+41

Manchester
Montreal
Moscow
Munich
Oslo
Ottawa
Paris
Prague
Quebec
Regina
Reykjavík
St. John's
Stockholm
Toronto
Vancouver
Vienna
Warsaw
Winnipeg
Zurich

+8
-6
+26
+14
+17
+3
+51
+2
-15
+58
+88
+1
-12
+18
+12
-5
-24
+29
+24

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 marked on the chart by a
symbol on the planets' event lines.
Here, conjunctions are considered to
occur when the planets actually appear
closest together in the sky (at appulse),
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
visible all night, occurs roughly when its
transit line crosses the Equation-of-time
line (not the line for midnight). Opposition is indicated there by a
symbol.
For instance, Saturn reaches opposition
on the night of August 1-2 this year.
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
Earth, respectively).
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 the dates
when their disks in telescopes show the
greatest illuminated extent in square
arcseconds. For example, Mercury does
so on the evening of January 20th and
Venus on December 4th this year.
Meteor showers are marked by a starburst symbol at the date of peak activity
and the time when the shower's radiant
is highest in the night sky. This is often
just as twilight begins before dawn.
(Note that we've adjusted the predicted
peak of the Southern Taurids, a sparse,
ill-deﬁned shower, to fall somewhat 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

Rising or Setting Corrections
Declination (North or South)
0°
5° 10° 15° 20° 25°

North Latitude

Skygazer's
Almanac 50°N

Other Charted Information

60°

1

11

23

36

53

80

55°

0

5

10

16

23

32

50°

0

0

0

0

0

0

45°

0

4

8

13

18

24

40°

1

8

15

23

32

43

35°

1

10

20

31

44

68

30°

1

12

25

39

54

72

25°

1

15

30

46

64

84

seven-digit 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 European observers this number applies all
night, because the next Julian day always
begins at 12:00 Universal Time (noon
Greenwich Mean Time).

Time Corrections
All events on this Skygazer's Almanac
are plotted for an observer at longitude
0° and latitude 50° north, a reasonable compromise for the countries of
northern and central Europe. However,
you need not be on a boat in the English
Channel 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.
To convert the charted time of an
event into your civil (clock) time, the
following corrections must be made.
They are given in decreasing importance:
* DAYLIGHT-SAVING TIME (OR "SUMMER
TIME"). When this is in effect, add one
hour to any time that you obtain 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
Europe are Greenwich Mean Time (or
Universal Time), 0°°; Central European

Time, 15°E; and Eastern European Time,
30°E. If your longitude is very close to one
of these (as is true for London), 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, Copenhagen (longitude
12.5° east) is 2.5° west of the Central
European Time meridian. So at Copenhagen, add 10 minutes to any time obtained
from the chart. The result is Central
European Standard Time.
Find your local-time correction and
memorize it. In the table at below left are
the corrections from local to standard
time, in minutes, for some major cities.
* RISING AND SETTING. Times of rising
and setting need correction if your latitude differs from 50° north. This effect
depends strongly on a star or planet's
declination. (The declinations of the Sun
and planets are listed in Sky & Telescope.)
If your site is north of latitude 50°,
then an object with a north declination stays above the horizon longer than
the chart shows (it rises earlier and sets
later), while one with a south declination spends less time above the horizon.
At a site south of 50°, the effect is just
the reverse. Keeping these rules in mind,
you can gauge roughly the number of
minutes by which to correct a rising or
setting time from the table above.
Finally, the Moon's rapid orbital
motion alters lunar rising and setting
times slightly if your longitude differs
from 0°. The Moon rises and sets about
two minutes earlier than the chart
shows for each time zone east of Greenwich Mean Time, and two minutes later
for each time zone west.

For reprints (item SGA21E, $5.95 each) or to order
a similar chart for latitude 40° 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.
http://www.shopatsky.com/resource-materials/calendars-almanacs http://www.shopatsky.com/resource-materials/calendars-almanacs
Sky & Telescope - January 2021

Table of Contents for the Digital Edition of Sky & Telescope - January 2021
