Sky and Telescope - January 2017 - SGA17S

Skygazer's 30°S
Almanac
FOR L ATITUDES
NE AR 30° SOUTH

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
2017, 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 8, 2017.
First find "January" and "8" 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 8-9 crosses many
SGA17S

2017

slanting event lines. Each event line tells
when something happens.
The dotted line for January 8-9 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 8th occurs at 7:06 p.m.
Local Mean Time. (All times read from the
chart are Local Mean Time, which can
differ from your standard clock time by
many minutes. More on this later.)
Moving to the right we see that at 8:35
p.m. the Pleiades transit the meridian,
meaning the famous star cluster is then
highest in the sky. Moreover, the sky is
almost fully dark, because evening twilight ends at 8:40 tonight when the Sun
is 18° below the horizon.
At 9:44 p.m. the brilliant planet
Venus, prominent in the western sky
until now, finally sets. At 10:10 the Large
Magellanic Cloud culminates (another
way of saying it transits) just as Mars
sets. Then the Orion Nebula (Messier
42) transits at 10:23, followed by the two
brightest nighttime stars, Canopus and
Sirius, at 11:11 and 11:33, respectively.
Transit times of such celestial landmarks
help us follow the march of constellations during the night.
The giant planet Jupiter rises at 11:50,
so it will be well up toward dawn.
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 8-9 this is 7h 13m. 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 8th the Sun runs
slow, transiting at 12:07 p.m. This variation is caused by the tilt of Earth's axis
and the ellipticity of its orbit.
At 1:54 a.m. we see a small Moon
symbol, and the legend at the chart's bottom indicates it is at gibbous phase, setting. (So we've had bright moonlight all
night until now.) Then at 2:07 Antares,
a star we usually associate with later
seasons, climbs above the horizon. The
ringed planet Saturn rises at 3:16.
The first hint of dawn - the start of
morning twilight - comes at 3:34 a.m.
Elusive Mercury rises soon thereafter, at
3:48, which is early enough before sunup
that we might spot it when it climbs
higher. The Sun finally peeks above the
eastern horizon at 5:08 a.m. on the
morning of January 9th.

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



Sky and Telescope - January 2017

Table of Contents for the Digital Edition of Sky and Telescope - January 2017

Contents
Sky and Telescope - January 2017 - Cover1
Sky and Telescope - January 2017 - Cover2
Sky and Telescope - January 2017 - 1
Sky and Telescope - January 2017 - Contents
Sky and Telescope - January 2017 - 3
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Sky and Telescope - January 2017 - Cover3
Sky and Telescope - January 2017 - Cover4
Sky and Telescope - January 2017 - SGA17RAL
Sky and Telescope - January 2017 - 90
Sky and Telescope - January 2017 - SGA17R
Sky and Telescope - January 2017 - SGA17RA
Sky and Telescope - January 2017 - SGA17EAL
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