Sky & Telescope - January 2021 - SA3

What's in the
sky tonight?
When does the Sun set, and when does
twilight end? Which planets are visible?
What time is moonrise?
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 30°° south -
in Australia, southern Africa, and the
southern cone of South America.
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
SGA21S

2021
FOR

L ATI TUDES

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 7:06
p.m. Local Mean Time. (All times read from
the chart are Local Mean Time, which can
differ from your civil clock time by many
minutes. More on this later.)
Note that Saturn, Mercury, and
Jupiter all set within a few minutes of 8
p.m., so they are close together in the sky,
but they'll be hard to spot so soon after
sunset. Moving to the right we see that
the Pleiades transit the meridian at 8:28
p.m., meaning the famous star cluster
is at its highest in the sky. Then twilight
ends at 8:40, marking the moment when
the Sun is 18° below the horizon and the
sky is fully dark.
At about 10:02 the Large Magellanic
Cloud culminates (another way of saying
it transits). Then the two brightest nighttime stars, Canopus and Sirius, transit
at 11:03 and 11:25, respectively. Transit
times of such celestial landmarks help us
follow the march of constellations 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 21m. 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 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 weav-

NE AR

30 °

SOU TH

ing 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 north. On January 10th the Sun runs slow, transiting
at 7 minutes past noon. This deviation, important for reading a sundial, is
caused by the tilt of the Earth's axis and
the ellipticity of its orbit.
At 12:07 a.m. the red planet Mars sets,
and we can infer that it has been visible
in the sky all evening. Dim Uranus sets
at 12:28. Then at 1:59 Antares, a star
we usually associate with later seasons,
climbs above the southeastern horizon.
At about 2:52 we notice a small Moon
symbol, and the legend tells us it is a
waning crescent, rising. The first hint of
dawn - the start of morning twilight -
comes at 3:36. Then brilliant Venus rises
at 3:50. The Sun finally peeks above the
eastern horizon at 5:10 a.m. on Monday
morning, January 11th.

Other Charted Information
Many of the year's most important
astronomical events are listed in the
chart's left-hand margin. Some are
marked on the chart itself.
Conjunctions (close pairings) of two
planets are marked by a
symbol on the

Local Mean Time Corrections
Adelaide
Brisbane
Canberra

+16
-13
+4

Melbourne
Perth
Sydney

+20
+18
-4

Cape Town
Durban
Harare

+46
-3
-4

Johannesburg +8
Port Elizabeth +18
Pretoria
+8

Asunción
-10
Buenos Aires +54
Montevideo +45

Rio de Janeiro -7
Santiago
+43
São Paulo
+6

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

South Latitude

Skygazer's
Almanac 30°S

planets' event lines. Here, the symbol
indicates the night when the planets
appear closest in the sky (at appulse), not
just when they have 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
transit line crosses the Equation-of-time
line (not the line for midnight). Opposition is marked there by a
symbol. For
instance, Saturn reaches opposition on
the night of August 2-3 this year.
Moonrise and moonset can be told
apart by whether the round limb - the
outside edge - of the Moon symbol faces
left (waxing Moon sets) or right (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 when their
telescopic disks have the greatest illuminated extent in square arcseconds. For
example, this occurs for Mercury on the
evening of January 20th and for Venus
on December 4th.
Meteor showers are marked by a starburst symbol on the date of peak activity
and at the time when the shower's
radiant (point of origin) is highest in
the night sky. This often occurs just as
morning twilight begins. (Note that our
predicted peak of the Southern Taurids, a
sparse and ill-defined shower, falls a few
weeks earlier than in recent years.)
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
days in January, add 215 to the date. For
instance, on January 10th we have 215 +
10 = 225, so the Julian day is 2,459,225.

10°

0

8

16

24

33

43

15°

0

6

12

19

26

33

20°

0

4

8

13

18

23

25°

0

2

4

7

9

12

30°

0

0

0

0

0

0

35°

0

2

5

7

10

13

40°

0

5

10

16

22

29

45°

1

8

17

26

37

49

50°

1

12

25

39

54

72

Note that the Julian day does not
change to this value until 12:00 Universal
Time (UT). In Australia, 12:00 UT falls
during the evening of the same day (at 10
p.m. Australian Eastern Standard Time,
AEST). Before that time, subtract 1 from
the Julian day number just obtained.

Time Corrections
All events on this southern version of
the Skygazer's Almanac are plotted for
an observer at longitude 135° east and
latitude 30° south. However, you need
not live near McDouall Peak, South
Australia, to use the chart. Simple corrections will allow you to get times accurate
to a couple of minutes anywhere in the
world's south 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 order of decreasing importance.
* DAYLIGHT-SAVING TIME ("SUMMER
TIME"). When this is in effect, add one
hour to any time read from the chart.
* YOUR LONGITUDE. The chart gives the
Local Mean Time (LMT) of events, which
differs from ordinary clock time by many
minutes at most locations. Our civil time
zones are standardized on particular longitudes. Examples in Australia are 150°°E
for the eastern states (which use Australian Eastern Standard Time, AEST), and
142.5°E for the central state and territory
(an odd value that puts the minute hands
of their clocks 30 minutes out of joint
with most of the rest of the world).
If your longitude is very close to your
standard time-zone meridian, luck is with
you and your LMT correction is zero. Oth-

erwise, 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, Melbourne, Australia
(longitude 145°), is 5° west of its timezone meridian (150°). So at Melbourne,
add 20 minutes to any time obtained from
the chart. The result is standard time.
Find your Local Mean Time correction
and memorize it; you will use it always.
The table below at left has the corrections,
in minutes, for some major cities.
* RISING AND SETTING. Times of rising
and setting need correction if your latitude differs from 30° south. This effect
depends strongly on a star or planet's
declination. The declinations of the Sun
and planets are listed each month in Sky
& Telescope.
If your site is south of latitude 30°S,
an object with a south declination stays
above the horizon longer than the chart
shows (it rises earlier and sets later), while
one with a north declination spends less
time above the horizon. If you are north of
30°S, the effect is just the reverse. With
these rules in mind, you can gauge the
number of minutes for correcting a rise or
set time using the table above left.
Finally, the Moon's rapid orbital
motion alters lunar rising and setting
times slightly if your longitude differs
from 135°E. The Moon rises and sets
about two minutes earlier than the chart
shows for each time zone east of central
Australia, and two minutes later for
each time zone west of there. Observers
in southern Africa can simply shift the
Moon symbol a third of the way to that
for the following date. Those in South
America can shift it about halfway there.

For reprints (item SGA21S, $5.95 each) or to order
a similar chart for latitude 40° north or 50° north,
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

Contents
Sky & Telescope - January 2021 - Cover1
Sky & Telescope - January 2021 - Cover2
Sky & Telescope - January 2021 - 1
Sky & Telescope - January 2021 - Contents
Sky & Telescope - January 2021 - 3
Sky & Telescope - January 2021 - 4
Sky & Telescope - January 2021 - 5
Sky & Telescope - January 2021 - 6
Sky & Telescope - January 2021 - 7
Sky & Telescope - January 2021 - 8
Sky & Telescope - January 2021 - 9
Sky & Telescope - January 2021 - 10
Sky & Telescope - January 2021 - 11
Sky & Telescope - January 2021 - 12
Sky & Telescope - January 2021 - 13
Sky & Telescope - January 2021 - 14
Sky & Telescope - January 2021 - 15
Sky & Telescope - January 2021 - 16
Sky & Telescope - January 2021 - 17
Sky & Telescope - January 2021 - 18
Sky & Telescope - January 2021 - 19
Sky & Telescope - January 2021 - 20
Sky & Telescope - January 2021 - 21
Sky & Telescope - January 2021 - 22
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Sky & Telescope - January 2021 - 24
Sky & Telescope - January 2021 - 25
Sky & Telescope - January 2021 - 26
Sky & Telescope - January 2021 - 27
Sky & Telescope - January 2021 - 28
Sky & Telescope - January 2021 - 29
Sky & Telescope - January 2021 - 30
Sky & Telescope - January 2021 - 31
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Sky & Telescope - January 2021 - 33
Sky & Telescope - January 2021 - 34
Sky & Telescope - January 2021 - 35
Sky & Telescope - January 2021 - 36
Sky & Telescope - January 2021 - 37
Sky & Telescope - January 2021 - 38
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Sky & Telescope - January 2021 - 42
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Sky & Telescope - January 2021 - 83
Sky & Telescope - January 2021 - 84
Sky & Telescope - January 2021 - Cover3
Sky & Telescope - January 2021 - Cover4
Sky & Telescope - January 2021 - SA1
Sky & Telescope - January 2021 - SA2
Sky & Telescope - January 2021 - SA3
Sky & Telescope - January 2021 - SA4
Sky & Telescope - January 2021 - SA5
Sky & Telescope - January 2021 - SA6
Sky & Telescope - January 2021 - SA7
Sky & Telescope - January 2021 - SA8
Sky & Telescope - January 2021 - SA9
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