Sky & Telescope - January 2023 - SA2

Almanac 2023
Skygazer's
30°S
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
2023, 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 15, 2023.
First find " January " and " 15 " 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 15-16 crosses
SGA23S
F OR LAT I T UD ES NE AR 30° SOUTH
many slanting event lines. Each event line
tells when something happens.
The dotted line for January 15-16
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 15th 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.)
Continuing rightward on the dotted
line for the 15th, we see that at 8:10
p.m. the Pleiades star cluster transits
the meridian, meaning it is due north
and highest in the sky. Then the planet
Venus sets at 8:20. Both of these events
occur when the sky is not yet fully dark.
Evening twilight doesn't truly end until
8:38, when the Sun is 18° below the horizon
(note the dashed line).
Moving to the right, we see that the
planet Mars transits the meridian at
8:45 and is very well placed for telescopic
study. But Saturn sets a minute later, so
it will be off tonight's observing list.
At about 9:45 the Large Magellanic
Cloud culminates (another way of saying
it transits). The two brightest nighttime
stars, Canopus and Sirius, transit at 10:45
and 11:07, respectively. Transit times of
such celestial landmarks help us follow
the nightly march of constellations.
A small Moon symbol is centered on
the dotted line at 11:53, and the legend
at the bottom of the chart tells us it is at
waning crescent phase, just rising.
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 15-16 this is 7h 39m. 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 15th
the Sun runs slow, transiting at 12:09
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.
At 12:36 a.m. the dim planet Uranus
sets. Then at 1:41 Antares, a star we usually
associate with later seasons, climbs
above the southeastern horizon. Five
minutes later, similarly hued Mars sets
in the west.
The first hint of dawn - the start of
morning twilight - comes at 3:41. The
elusive planet Mercury rises at 4:06. The
Sun finally peeks above the southeastern
horizon at 5:14 a.m. on Monday morning,
January 16th.
Other Charted Information
Many of the year's most important
astronomical events are listed in the
Local Mean Time Corrections
Adelaide
Brisbane
Canberra
+16
-13
+4
Cape Town +46
Durban
Harare
Asunción
-3
-4
-10
Buenos Aires +54
Montevideo +45
Melbourne
Perth
Sydney
+20
+18
-4
Johannesburg +8
Port Elizabeth +18
Pretoria
+8
Rio de Janeiro -7
Santiago
São Paulo
+43
+6
Sky & Telescope - January 2023

Table of Contents for the Digital Edition of Sky & Telescope - January 2023
