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=head1 SPECIFYING ANGLES

This script accepts angle input in the following formats:

* Decimal degrees.

* Hours, minutes, and seconds, specified as hours:minutes:seconds. You
would typically only use this for right ascension. You may specify
fractional seconds, or fractional minutes for that matter.

* Degrees, minutes, and seconds, specified as degreesDminutesMsecondsS.
The letters may be specified in either case, and trailing letters may
be omitted. You may specify fractional seconds, or fractional minutes
for that matter.

Examples:

 23.4 specifies 23.4 degrees.
 1:22.3 specifies an hour and 22.3 minutes
 12d33m5 specifies 12 degrees 33 minutes 5 seconds

Right ascension is always positive. Declination and latitude are
positive for north, negative for south. Longitude is positive for
east, negative for west.

=head1 SPECIFYING TIMES

This script (or, more properly, the modules it is based on) does not,
at this point, do anything fancy with times. It simply handles them as
Perl scalars, with the limitations that that implies.

Times may be specified absolutely, or relative to the previous absolute
time, or to the time the script was invoked if no absolute time has
been specified.

Both absolute and relative times may contain whitespace. If they do,
they need to be quoted. For example,

 satpass> pass today +1

needs no quotes, but

 satpass> pass 'today midnight' '+1 12'

needs quotes.

=head2 Absolute time

Any time string not beginning with '+' or '-' is assumed to be an
absolute time. If L<Date::Manip|Date::Manip> is available, the string is
fed to that for parsing. See the documentation for that module for all
the possibilities. Some of them are:

 today        'today noon'        'next monday'
 tomorrow     'yesterday 10:00'   'nov 10 2:00 pm'

L<Date::Manip|Date::Manip> has at least some support for locales, so
check L<Date::Manip|Date::Manip> before you assume you must enter dates
in English.

If C<Date::Manip> is not available, this script will do the best it can
with an internal parsing routine. This routine accepts ISO-8601 dates,
but not ordinal day specifications (e.g. 2009365 for December 31 2009)
or week specifications (e.g. 2009W0101 for January 4 2009 (if that is in
fact the correct interpretation of the spec)).

The internal routine is rather permissive about punctuation: any
non-digit character is accepted, and multiple whitespace characters are
accepted between date and time, and between time and zone. Years can be
two- or four-digit, with two-digit years representing years between 1970
and 2069, inclusive. Any other date or time field can be shortened if it
has trailing punctuation or is the last field specified before the zone.

In addition, the internal routine accepts the strings 'yesterday',
'today', and 'tomorrow' in lieu of a date. If time is not specified,
these represent midnight.

So, using the internal parser the following are valid:

 today     'today 12:00'  '2009/2/1 6:00Z' (i.e. February 1)
 tomorrow  yesterday6:00

As a refresher, ISO-8601 dates are numeric and specified as year, month,
and day. If all fields are full-width (4 digits for years, 2 for
everything else) no punctuation at all is needed, other than in an
optional zone specification.

=head2 Relative time

A relative time is specified by '+' or '-' and an integer number of
days. The number of days must immediately follow the sign. Optionally,
a number of hours, minutes, and seconds may be specified by placing
whitespace after the day number, followed by hours:minutes:seconds. If
you choose not to specify seconds, omit the trailing colon as well. The
same applies if you choose not to specify minutes. For example:

+7 specifies 7 days after the last-specified time.

'+7 12' specifies 7 days and 12 hours after the last-specified time.

If a relative time is specified as the first time argument of a command,
it is relative to the most-recently-specified absolute time, even if
that absolute time was specified by default. Relative times in subsequent
arguments to the same command are relative to the previously-specified
time, whether absolute or relative. For example:

 almanac '' +5

establishes the most-recently-specified time as 'today midnight', and does
an almanac for 5 days from that time. If the next command is

 almanac +5 +3

this produces almanac output for three days, starting 5 days after
'today midnight'.

=head1 INVOCATION