Sky & Telescope - January 2025 - SGA3

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
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, Mars reaches opposition on the
night of January 15-16 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 Venus on the
evening of February 14th 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
(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 2460,
as indicated just off the chart's upper left
margin. To find the last three digits for
days in January, add 676 to the date. For
instance, on January 5th we have 5 + 676
= 681, so the Julian Day is 2,460,681.
Rising or Setting Corrections
Declination (North or South)
0° 5° 10° 15° 20° 25°
10° 0
15° 0
20° 0
25° 0
30° 0
35° 0
40° 0
45° 1
50° 1
8
6
4
2
2
5
8
16 24
12 19
8
4
5
13
7
7
12 25
10 16
17 26
39
33 43
26 33
18 23
9 12
10 13
22 29
37 49
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. 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 on
the Planetary Almanac page of 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 who live
in South America can shift the symbol
about halfway there.
For reprints (item SGA25S) or to order a similar
chart for latitude 40° north or 50° north, go to:
shopatsky.com/collections/maps-globes/almanacs
Skygazer's Almanac 2025 is a
supplement to Sky & Telescope
Magazine, 1374 Massachusetts
Avenue, Cambridge, MA 02138,
USA, skyandtelescope.org.
©2024 AAS Sky Publishing,
LLC. All rights reserved.
South Latitude
http://www.shopatsky.com/collections/maps-globes/almanacs http://www.skyandtelescope.org
Sky & Telescope - January 2025

Table of Contents for the Digital Edition of Sky & Telescope - January 2025
