Sky and Telescope - January 2016 - S8

Time Corrections
All events on this Skygazer's Almanac
are plotted for an observer at 90° west
longitude and 40° north latitude, near
the population center of North America.
However, you need not live near Peoria,
Illinois, to use the chart. Simple corrections will allow you to get times accurate to a couple of minutes anywhere in
the world's north temperate latitudes.
To convert the charted time of an

Rising or Setting Corrections
0°

North Latitude

Jupiter on the night of March 7-8.
Moonrise and moonset can be told
apart by whether the round limb - the
outside edge - of the Moon symbol
faces right (waxing Moon sets) or left
(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).
The elusive planet Mercury never
strays far outside the twilight bands. Its
dates of greatest elongation from the Sun
are shown by ◗ symbols on its rising or
setting curves. Asterisks mark the dates
when it shows its greatest illuminated
extent in square arcseconds. (The same
symbols can also appear on Venus's
curves, but not in 2016.)
Meteor showers are marked by a starburst symbol on the date of peak activity
and at the time when the shower's radiant is highest in the night sky. This is
often 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
seven-digit 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
evenings in January, add 388 to the date.
For instance, on the evening of January
17th we have 388 + 17 = 405, so the Julian
day is 2,457,405. For North American
observers this number applies all night,
because the next Julian day always
begins at 12:00 Universal Time (6:00
a.m. Central Standard Time).

Declination (North or South)
5° 10° 15° 20° 25°

50°

45°

40°

35°

30°

25°

event to your civil (clock) time, the following corrections must be made. They
are listed in decreasing importance:
* daylight-saving time. When this
is in eﬀect, add one hour to any time
obtained from the chart.
* your longitude. The chart gives
the Local Mean Time (LMT) of events,
which diﬀers from ordinary clock time
by a number of minutes at most locations. Our civil time zones are standardized on particular longitudes. Examples
in North America are Eastern Time, 75°
W; Central, 90°; Mountain, 105°; and
Paciﬁc, 120°. If your longitude is very
close to one of these (as is true for New
Orleans and Denver), luck is with you
and this correction is zero. Otherwise,

Local Mean Time Corrections
Atlanta
Boise
Boston
Buffalo
Chicago
Cleveland
Dallas
Denver
Detroit
El Paso
Helena
Honolulu
Houston
Indianapolis
Jacksonville
Kansas City

+38
+45
-16
+15
-10
+27
+27
0
+32
+6
+28
+31
+21
+44
+27
+18

Los Angeles
Memphis
Miami
Minneapolis
New Orleans
New York
Philadelphia
Phoenix
Pittsburgh
St. Louis
Salt Lake City
San Francisco
Santa Fe
Seattle
Tulsa
Washington

-7
0
+21
+13
0
-4
+1
+28
+20
+1
+28
+10
+4
+10
+24
+8

Athens
Baghdad
Beijing
Belgrade
Cairo
Istanbul
Jerusalem

+25
+3
+14
-22
-8
+4
-21

Lisbon
Madrid
New Delhi
Rome
Seoul
Tehran
Tokyo

+36
+75
+21
+10
+32
+4
-19

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, Washington, DC (longitude 77°), is 2° west of the Eastern Time
meridian. So at Washington, add 8 minutes to any time obtained from the chart.
The result is Eastern Standard Time.
Find your time adjustment and
memorize it. The table below shows the
corrections from local to standard time,
in minutes, for some major cities.
* rising and setting. Times of rising
and setting need correction if your latitude diﬀers from 40° north. This eﬀect
depends strongly on a star or planet's
declination. (The declinations of the Sun
and planets are given in each month's
Sky & Telescope.)
If your site is north of latitude 40°,
then an object with a north declination
stays above the horizon longer than the
chart shows (it rises earlier and sets
later), whereas one with a south declination spends less time above the horizon.
At a site south of 40°, 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 90° west. The Moon rises and sets
about two minutes earlier than the chart
shows for each time zone east of Central
Time, and two minutes later for each
time zone west of Central Time. European observers can simply shift each
rising or setting Moon symbol leftward
a quarter of the way toward the one for
the previous night.
Skygazer's Almanac 2016 is a supplement to
Sky & Telescope. © 2016 F+W Media, Inc.
All rights reserved.
For reprints (item SGA16R, $4.95 each postpaid) or to
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Sky and Telescope - January 2016

Table of Contents for the Digital Edition of Sky and Telescope - January 2016