Sky & Telescope - January 2020 - SA2

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
2020, 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 12, 2020.
First ﬁnd "January" and "12" 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 12-13 crosses
SGA20S

many slanting event lines. Each event line
tells when something happens.
The dotted line for January 12-13
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 12th occurs at 7:05
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 both Saturn and Mercury
set within a few minutes of the Sun, so
we can cross them off tonight's observing list. Moving to the right we see that
the Pleiades transit the meridian at 8:23
p.m., meaning the famous star cluster
is at its highest in the sky. We see a tiny
Moon symbol centered at 8:32, and the
legend at the chart's bottom tells us the
Moon is at waning gibbous phase, rising.
Then twilight ends at 8:40, marking the
moment when the Sun is 18° below the
horizon and the sky is fully dark.
Venus sets at 9:14 p.m., and we can
infer that the brilliant planet has up to
now been sinking low in the western sky.
At about 9:57 the Large Magellanic
Cloud culminates (another way of saying
it transits). The Orion Nebula (Messier
42) transits at 10:10, and then the two
brightest nighttime stars, Canopus and
Sirius, transit at 10:58 and 11:20, 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 12-13 this is 7h 26m. 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 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 north. On January 12th the Sun runs
slow, transiting at 12:08 p.m. This deviation, important for reading a sundial, is
caused by the tilt of Earth's axis and the
ellipticity of its orbit.
At 12:11 a.m. the faint planet Uranus
sets. At 1:54 Antares, a star we usually associate with later seasons, climbs
above the southeastern horizon. Then
Mars comes up 3 minutes later.
The ﬁrst hint of dawn - the start of
morning twilight - comes at 3:37. Then
the bright planet Jupiter rises at 4:14.
The Sun ﬁnally peeks above the eastern
horizon at 5:11 a.m. on Monday morning, January 13th.

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.

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

F O R L ATITUDES
FOR
ATIT U D ES
NE A R 30° S OU TH

Conjunctions (close pairings) of two
planets are marked by a symbol on the
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 July 20-21 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, and asterisks mark when
their telescopic disks have the greatest
illuminated extent in square arcseconds.
For example, this occurs for Mercury
on the evening of February 5th and for
Venus April 28th.
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.
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 early this year are 2458,
as indicated just off the chart's upper left
margin. To ﬁnd the last three digits for
days in January, add 849 to the date. For
instance, on January 12th we have 849 +
12 = 861, so the Julian day is 2,458,861.
Note that the Julian day doesn't

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

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. Eastern Standard Time, EST).
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 135° east longitude and 30°
south latitude. 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 Eastern
Standard Time, EST), and 142.5° E for
the two central states (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. 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, 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.

Skygazer's Almanac 2020 is a supplement to Sky &
Telescope magazine. ©2020 AAS Sky Publishing, LLC.
All rights reserved.
For reprints (item SGA20S, $5.95 each postpaid)
or to order a similar chart for north latitude
40° or 50°, contact
Sky & Telescope,
One Alewife Center,
Cambridge, MA 02140, USA.
Send an e-mail
to skyprodservice@
skyandtelescope.com,
or visit shopatsky.com.
Sky & Telescope - January 2020

Table of Contents for the Digital Edition of Sky & Telescope - January 2020
