Sky and Telescope - January 2017 - SGA17R

Skygazer's 40°N
Almanac
FOR L ATITUDES
NE AR 40° NORTH

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
2017, 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, 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 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
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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 4:52 p.m.
Local Mean Time. (All times on the chart
are Local Mean Time, which can differ
from your standard clock time. More on
this later.)
Moving to the right, we see that
evening twilight ends at 6:29 p.m., the
time when the Sun is 18° below the
horizon and the sky is fully dark. Then
at 7:39 Polaris, the North Star, reaches
upper culmination. This means it stands
directly above the north celestial pole (by
40′ this year), a good time to check the
alignment of an equatorial telescope.
At 8:33 p.m. the Pleiades transit the
meridian, meaning the famous star cluster is due south and highest in the sky. At
8:46 the brilliant planet Venus sets in the
west, followed by dim Neptune at 9:06
and red-orange Mars at 9:35.
The Great Orion Nebula (Messier 42)
transits at 10:21, as does the bright star
Sirius at 11:30. Transits of such celestial
landmarks help indicate when they are
best placed for viewing, and where the
constellations are 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 8-9 this is 7h 16m. 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 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.
Giant planet Jupiter rises at 12:29
a.m. and will become better placed for
telescopic viewing toward dawn.
At 4:06 a.m. we see a Moon symbol,
and the legend at the chart's bottom
tells us it is at waxing gibbous phase,
and setting. (So the night until now has
been brightly moonlit.) Then at 4:49 a.m.
Antares, a star we usually associate with
a much later season, rises.
The ringed planet Saturn comes up
at 5:26, and the ﬁrst hint of dawn - the
start of morning twilight - comes at
5:45. A few minutes later elusive Mercury
rises, early enough before sunup that we
should spot it later as it climbs higher.
The Sun ﬁnally peeks above the horizon
at 7:22 a.m. on January 9th.

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 on the chart by a
symbol on the planets' event lines.
Here, conjunctions are considered to
occur when the planets actually appear
closest together 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 when its transit
line crosses the Equation-of-time line
(not the line for midnight). Opposition is
marked there by a
symbol, as is done
Sky and Telescope - January 2017

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