My first Magazine Sky & Telescope - 01.2019 | Page 91

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 correc-
tions will allow you to get times accu-
rate to a couple of minutes anywhere in
the world’s north temperate latitudes.
Rising or Setting Corrections
Declination (North or South)
tion is marked there by a
symbol, as
for Saturn on the night of July 8–9.
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 (wan-
ing Moon rises). Or follow the nearly
horizontal row of daily Moon symbols
across the chart to fi nd 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 from
the twilight bands. Their dates of greatest
elongation from the Sun are shown by ◗
symbols on their rising or setting curves.
Asterisks mark their dates of greatest illu-
minated extent in square arcseconds. For
example, this occurs for Mercury on the
evenings of February 20th and June 18th,
but not at all for Venus this year.
Meteor showers are marked by a star-
burst symbol on the date of peak activity
and at the time when the shower’s radi-
ant 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 2458, as
indicated just off the chart’s upper left
margin. To find the last three digits for
evenings in January, add 484 to the date.
For instance, on the evening of January
6th we have 484 + 6 = 490, so the Julian
day is 2,458,490. 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. Cen-
tral Standard Time).
0° 5° 10° 15° 20° 25°
50° 0 7 14 23 32 43
45° 0 3 7 10 14 19
40° 0 0 0 0 0 0
35° 0 3 6 9 12 16
30° 0 5 11 16 23 30
25° 0 8 16 24 32 42
To convert the charted time of an
event to your civil (clock) time, the fol-
lowing corrections must be made. They
are mentioned in order of 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
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
–7
Memphis
0
Miami
+21
Minneapolis +13
New Orleans
0
New York
–4
Philadelphia
+1
Phoenix
+28
Pittsburgh
+20
St. Louis
+1
Salt Lake City +28
San Francisco +10
Santa Fe
+4
Seattle
+10
Tulsa
+24
Washington
+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
Orleans and Denver), luck is with you
and this 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 min-
utes for each degree you are east of it.
For instance, Washington, DC (longi-
tude 77°), is 2° west of the Eastern Time
meridian. So at Washington, add 8 min-
utes to any time obtained from the chart.
The result is Eastern Standard Time.
Find your time adjustment and memo-
rize it. The table below at left shows the
corrections from local to standard time,
in minutes, for some major cities.
• RISING AND SETTING . These times need
correction if your latitude differs from
40° north. This effect depends strongly
on a star or planet’s declination (listed
monthly on the Planetary Almanac page
of Sky & Telescope).
If your site is north of latitude 40°,
then an object with a north declina-
tion stays above the horizon longer than
the chart shows (it rises earlier and sets
later), whereas one with a south declina-
tion 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 affects lunar rising and setting
times 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 it. European observers can simply
shift each rising or setting Moon symbol
leftward a quarter of the way toward the
one for the previous night.
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