Harrowsmith Fall 2019 | 157ASTRONOMYzone, and the latitude difference
between Ottawa and your city. To
your initial time zone correction from
eastern time, add the number in the
table corresponding to the month and
your city, or subtract it if the number
is negative.
The entries are given for mid-
month, so you may wish to interpolate
to specific days for more accurate
values. There will be additional errors
due to interpolation of the data if
you are not in the city centre or at
ground level; however, these will be
relatively small.
MONTHLY TABLES
These tables provide the times of
numerous celestial events. To save
space, the sun and moon information
and other celestial events are listed
using only Ottawa time, which adopts
daylight time at 2 a.m. on March 8
until November 1 at 2 a.m. Those
living in Saskatchewan and a few
other local areas must convert these
times back to standard time. Sorry.
Sunrise and sunset columns
Sunrise is defined as the moment
when the upper edge of the sun first
clears the horizon; sunset is timed
when the upper limb disappears. The
published times are for a level horizon
unobstructed by terrain, trees or
buildings. We have already applied an
average correction for atmospheric
refraction (about half a degree), which
makes sunrise earlier and sunset
later, but the actual amount depends
on the air density on that day and is
not significant for a casual stargazer.
Although the sun may be below the
horizon, it still illuminates the high-
altitude atmosphere. This causes the
lingering light of dusk and dawn. The
beginning and end of these are included
in the tables as “civil twilight.” At the
end of civil twilight, the sky will be dark
enough for casual observing.
We list the azimuth angle of sunrise
and sunset, where “azimuth” is defined
as the angle from true north around the
horizon toward the east (i.e., east is
90 degrees azimuth). The altitude of the
sun at midday is in degrees above the
southern horizon as seen from Ottawa.
You will have to make corrections
for the latitude of your city: take the
difference in the degrees of latitude
of your city from Ottawa (45 degrees),
which will be positive if you are south
and negative if you are north; add
this number to the value in the
“Alt.” column.
Note that the direction north
indicated by a compass refers to
magnetic north, which is not the same
as true north. If you are using a compass
to get your bearings, you need to correct
the direction by adding or subtracting
your local magnetic variation, which
can be determined from geomag.nrcan.
gc.ca/calc/mdcal-en.php.Moonrise and
moonset columns
The definitions for the “moon”
columns are similar to those for
the “sun” columns. Moonrise and
moonset times are for Ottawa, but
there is no simple way of applying
them to other places because the
moon drifts in position with respect
to the stars by its own diameter