Sky and Telescope - January 2017 - SGA17SA

Rising or Setting Corrections
Declination (North or South)
0°
5° 10° 15° 20° 25°

South Latitude

Conjunctions (close pairings) of two
planets are indicated by a
symbol on
the planets' event lines. Here, conjunctions are indicated 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 when its transit
line crosses the Equation-of-time line
(not the line for midnight). Opposition is marked there by a
symbol. For
instance, Jupiter reaches opposition on
the night of April 7-8 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
outside the twilight bands. Their dates
of greatest elongation from the Sun are
shown by ◗ symbols on their rising or
setting curves, and asterisks mark when
their telescopic disks have the greatest
illuminated extent in square arcseconds.
In the case of Venus, this is also very
nearly the date when it reaches greatest
brilliancy, as on February 17th 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 this year are 2457, as
indicated just off the chart's upper left
margin. To ﬁnd the last three digits for
days in January, add 754 to the date. For
instance, on January 8th we have 754 + 8
= 762, so the Julian day is 2,457,762.
Note that the Julian day doesn't

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50°

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. Eastern Standard Time, EST).
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 135° east longitude and 30°
south latitude. 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.
i DAYLIGHT-SAVING TIME ("SUMMER
TIME"). When this is in effect, add one
hour to any time read from the chart.
i 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 Eastern
Standard Time, EST), and 142.5° E for
the two central states (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. You can look up your
longitude on a map.
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 far left has the corrections, in minutes, for some major cities.
i 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 in Sky & Telescope.
If your site is south of latitude 30° S,
then 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. At a site north
of 30° S, 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 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 in South
America can shift it about halfway there.
Skygazer's Almanac 2017 is a supplement to
Sky & Telescope. ©2017 F+W Media, Inc.
All rights reserved.
For reprints (item SGA17S, $5.95 each postpaid)
or to order a similar chart for north latitude 40° or
50°, contact Sky & Telescope, 90 Sherman St., Cambridge, MA 02140, USA; phone +1 617-864-7360,
fax +1 617-864-6117. Send an
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Sky and Telescope - January 2017

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