AstroInfo   Astronomical Information

by Martin Dale


PURPOSE
The purpose of this application is to provide details of astronomical data
for the Sun, Moon and planets from any given longitude and latitude. It can
also be used to determine the day of the week on which a particular date
falls and the date of Easter Sunday.

GETTING STARTED
To use AstroInfo, double-click on its icon in a directory viewer to
install the application on the icon bar.

OPTIONS
Clicking Menu over the icon bar icon will display a menu of options.

Info
Move right across the Info arrow to open the standard information window.

Settings
Selecting Settings will open the data entry window which will enable you
to enter details of the time and date required and your longitude and
latitude. These are adjusted by clicking Select or Adjust over the arrow
icons. The Adjust button will change the selected setting in the opposite
direction to the arrow. As the data is changed the day of the week will
automatically be updated. When the Settings window is first opened the
current time and date are read from the system clock and the observer's
longitude and latitude are read from the text file, Coords.

The file Coords contains details of the observer's longitude and latitude.
The co-ordinates can be altered by loading the file into Edit and changing
the details shown. The file must contain just 5 lines of text, the first
four show the observer's longitude, then latitude in both degrees and
minutes with the final line containing the longitude direction with respect
to the Greenwich meridian. This will be either "E" for East of the meridian
or "W" for West. These details can be found from most maps. As a guide the
co-ordinates of some locations in Britain are shown below:

Location Longitude Latitude

Greenwich 00 00' E  51 28' N
Belfast 05 56' W  54 37' N
Cardiff 03 10' W  51 28' N
Glasgow 04 17' W  55 52' N
Liverpool 02 57' W  53 24' N
Manchester 02 14' W  53 29' N
Oxford 01 15' W  51 45' N

Once the correct settings have been selected click on the OK icon and all
open windows will be updated. Clicking on Reset will set the co-ordinates
to those initially read in from the Coords file and the time and date will
be read from the system clock. Selecting Cancel will close the window,
cancelling any changes since the window was opened or the OK icon was
selected.

Sun
Select Sun to open the Sun window. This shows times of Sunrise, Sunset
and Civil Twilight, the Sun-Earth distance, the Sun's angular diameter and
the date of Easter Sunday in the selected year. Any attempt to calculate the
Sunrise and Sunset times at latitudes and dates when the sun either
doesn't rise or set (circumpolar) will produce a warning, though this will
only be displayed if the Sun window is open. The Sunrise and Sunset
times will then be marked as not available. Where Twilight lasts all night,
the times will be similarly marked.

The date of Easter Sunday is given by the first Sunday after the first full
moon which falls upon or after 21st March. The algorithm used to calculate
this is only accurate for the Gregorian calendar which was ordained by Pope
Gregory in October 1582 though it wasn't adopted by Britain until 14th
September 1752.

Moon
Details of the moon are displayed by selecting Moon from the icon bar
menu. These show its rising and setting times, angular size, distance from
the Earth and phase. As with the Sun, an error window is displayed when the
Moon window is open if the selected date and latitude results in the Moon
being circumpolar or not rising above the horizon on the chosen day. The
Moon's phase is the ratio of the area of the illuminated part of the Moon to
the entire disc. A phase of 1 represents a full Moon and one of 0 indicates
a new Moon. The interval between successive new Moons, known as the Synodic
month is 29 days 12 hrs 44 mins. The distance of the Moon varies between
356,000 Km at perigee, the closest point to the Earth, and 407,000 Km at
apogee, the furthest point.

Planets
Clicking Select on the Planet menu option opens a window providing details
of all the planets, excluding the Earth. Details on individual planets can
then be chosen by clicking Select or Adjust over the arrow icons. The
details provided include the planet's right ascension and declination,
distance from the Earth, angular size, phase and light travel time.

Right ascension and declination define the position of a planet in the sky
and are the equivalent of longitude and latitude on the Earth. If we imagine
the plane of the Earth's equator extended indefinitely it forms the
celestial equator. The angular distance from a planet perpendicular to the
celestial equator is the planets declination. This is normally measured in
degrees, positive north of the celestial equator and negative south of it.
Right ascension is slightly more difficult to define as a baseline has to be
chosen, the equivalent of the Greenwich meridian at 0 longitude. The path
of the Earth around the Sun, known as the ecliptic, forms an angle of about
23 26' to the celestial equator, therefore crossing it in two places. One
of these, known as the first point of Aries or the Vernal Equinox, is taken
as being the reference point in defining right ascension which is measured
eastwards from this point along the celestial equator. Right ascension can
be measured in degrees but it is more common to measure it in hours, minutes
and seconds of time from 0 to 24 hours. One complete revolution of 360
corresponds to 24 hours, thus 1 hour is equivalent to 15.

The distance of a planet is that measured from the Earth and is given in
Astronomical Units (AU) where 1 AU is the mean distance from the Earth to
the Sun, 149.6 million Km. The light travel time is the time taken for the
light to reach us from the planet.

Quit
Clicking on Quit will remove AstroInfo from the icon bar.


 Copyright RISC User 1993
