Sky & Telescope - January 2020 - SA5

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
2020, 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 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
SGA20R

2020

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 4:56 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.)
We also note that Mercury has just set
and that Saturn sets 1 minute after the
Sun. We can cross those two planets off
tonight's observing list. Moving to the
right, we see that the brightest nighttime
star, Sirius, rises at 6:13 p.m. in the deepening twilight. A dashed line shows that
twilight technically ends at 6:32, the time
when the Sun is 18° below the horizon.
The faint planet Uranus transits the
meridian at 6:35, meaning it is due south
and "riding high," an excellent time to
look for it in binoculars. Not long after,
at 7:15, a tiny Moon symbol appears on
the dotted line, and the legend at the bottom of the chart tells us it is at waning
gibbous phase, rising.
At 7:30 Polaris, the North Star,
reaches upper culmination. This means it
stands directly above the north celestial
pole (by 39′ this year), a good time to
check the alignment of an equatorial
telescope.
Venus sets at 7:57 p.m., so we infer it
has been shining brightly up to now, low
in the western sky. At 8:20 the Pleiades
star cluster transits, followed at 10:08 by
the Orion Nebula, M42. Transits of such
celestial landmarks help indicate when
they are best placed for viewing.
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 29m. 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 12th the Sun runs
slow, transiting at 12:08 p.m. This deviation is important for reading a sundial.
It is caused by the tilt of Earth's axis and
the ellipticity of its orbit.
The red planet Mars rises at 4:03. It
is followed up at 4:36 by Antares, 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.
And this morning Jupiter rises at 6:32.
The Sun ﬁnally peeks above the horizon
at 7:21 a.m. on January 12th.

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
(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 thus
visible all night, occurs roughly when its
transit line crosses the Equation-of-time
line (not the line for midnight). Opposi-

Time Corrections
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)
0°
5° 10° 15° 20° 25°

North Latitude

F O R L ATITUDES
FOR
ATIT U D ES
S
NE A R 40° N O R TH

tion is marked there by a
symbol, as
for Saturn on the night of July 20-21.
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 their dates of greatest
illuminated extent in square arcseconds.
For example, this occurs for Mercury on
the evening of February 5th and for Venus
on April 27th 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.
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 evenings in January, add 849 to the
date. For instance, on the evening of
January 12th we have 849 + 12 = 861, so
the Julian day is 2,458,861. 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).

50°

45°

40°

35°

30°

25°

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
importance:
* 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 Orleans and

Local Mean Time Corrections
Atlanta
Boise
Boston
Buffalo
Chicago
Cleveland
Dallas
Denver
Detroit
El Paso
Helena
Honolulu
Houston
Indianapolis
Jacksonville
Kansas City

+38
+45
-16
+15
-10
+27
+27
0
+32
+6
+28
+31
+21
+44
+27
+18

Los Angeles
-7
Memphis
0
Miami
+21
Minneapolis +13
New Orleans
0
New York
-4
Philadelphia
+1
Phoenix
+28
Pittsburgh
+20
St. Louis
+1
Salt Lake City +28
San Francisco +10
Santa Fe
+4
Seattle
+9
Tulsa
+24
Washington
+8

Athens
Baghdad
Beijing
Belgrade
Cairo
Istanbul
Jerusalem

+25
+3
+14
-22
-8
+4
-21

Lisbon
Madrid
New Delhi
Rome
Seoul
Tehran
Tokyo

+36
+75
+21
+10
+32
+4
-19

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 at 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 (listed
monthly on the Planetary Almanac page
of 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.
Skygazer's Almanac 2020 is a supplement to Sky &
Telescope magazine. ©2020 AAS Sky Publishing, LLC.
All rights reserved.
For reprints (item SGA20R, $4.95 each postpaid) or
to order a similar chart for latitude 50° north or 30°
south, contact Sky & Telescope,
One Alewife Center,
Cambridge, MA 02140, USA;
phone 800-253-0245. Send
an e-mail to skyprodservice
@skyandtelescope.com, or
you can visit our online
store at shopatsky.com.

Sky & Telescope - January 2020

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