The menu also lets you control whether sunlighting is calculated and
shown, and whether full 3-dimensional (3-D) shading is to be used. You
can also change the twilight setting and the background colour.
- Daylight image
- displays a sub-menu with the following options for controlling the
image seen on the daylight side of the globe (or for the whole globe if
sunlighting is switched off):
- Classic
- uses the ‘Classic’ PMGlobe land and sea image (a solid colour for
each)
- Default (full colour)
- uses the built-in full-colour NASA Visible Earth[1]
daylight image
- Image from file
- uses an image from a selected file for the daylight side of the
globe (see the next menu choice for details); this option is only
available after a file has been selected
- Set daylight image file...
- chooses (using the Windows file selection dialog) an image which is
then automatically selected for display instead of the Classic or
Default images.
The image selected should normally be a cylindrical projection of the
Earth, twice as wide as high, with the intersection of the Greenwich
Meridian (0° longitude) and the Equator (0° latitude) in the
centre. It must have a file extension (type)
of .jpg, .gif, or .bmp. For best results it should
be at least 900 pixels high.
If any other image or projection is used, it will be projected onto the
surface of the globe as though it were a cylindrical projection.
Whenever the image is used to refresh the globe, it will be reloaded
if its timestamp has changed since the last use – any update to the
image file will therefore, in due course, appear on the globe.
- Classic land colour
- displays a sub-menu which lets you select the colour of the land
shown when using the Classic daylight image
- Classic water (lake and sea) colour
- displays a sub-menu which lets you select the colour of water
features shown when using the Classic daylight image.
- Night image
- displays a sub-menu with the following options for controlling the
image seen on the night side of the globe (when sunlighting is
selected):
- Classic (starlight)
- uses the ‘Classic’ PMGlobe land and sea image to create a
‘starlight’ effect (black sea, faint grey land)
- Default (city lights)
- uses the built-in NASA Visible Earth night-time image
- Image from file
- uses an image from a selected file (see below) for the night side of
the globe; this option is only available after a file has been selected
- Reduced daylight
- the currently selected daylight image is used, somewhat darkened,
for the night side of the globe
- Dark daylight
- the currently selected daylight image is used, much darkened, for
the night side of the globe
- None (pitch black)
- no image is shown for the night side of the globe
- Set night image file...
- chooses (using the Windows file selection dialog) an image which is
then automatically selected for night-time display instead of the
Classic or Default images.
The night-time image is selected and used in the same way as the
daylight image from file (see above).
- Sunlight
-
When checked, the globe is displayed as though lit by sunlight at the
current time of day, with the daylight image being used for the
side of the globe facing the sun and the night-time image for the
obverse. The images are mixed in the twilight zone.
This lets you see at a glance which parts of the world are in daylight,
and which are in the dark. This lighting is (of course) time dependent,
so your PC must have the correct time of day and time zone set in order
to show the sunlight correctly (right-click on the clock on the Windows
task bar to adjust these, or use the Date and Time option in the Control
Panel).
If sunlight is not selected, the daylight image will be used for the
whole globe.
When sunlight is being shown, the globe will normally be re-drawn
(refreshed) at regular intervals, as selected from the Refresh
option in the Options menu. This shows how
the sunlight moves across the earth.
The section on accuracy gives more details.
- 3-D
-
This choice adds shading, to give the globe a ‘three-dimensional’
(3-D) appearance. When using sunlight, the shading used indicates the
lighting effect of the sun on the earth (and hence is brightest where
the sun is overhead, and somewhat darker near the terminator).
If 3-D is selected without selecting sunlight, artificial lighting from
the front and top left of the globe is supplied, as though the globe
were being lit by overhead room lighting.
- Twilight setting
-
displays a sub-menu which lets you choose how the twilight band is
shown on the globe, when sunlight is in effect.
The sun is not a point source of light, and the atmosphere scatters its
light, too. Therefore, sunlight is seen on the dark side of the earth
for some time before the sun rises and for some time after it sets. You
can select to see all, part, or none of this twilight band on the globe.
The twilight band, when shown, is displayed as a graded mix of the
daylight and night-time images.
Beyond the selected band, the night-time image is used.
You may choose any one of the twilight settings:
- Theoretical sunrise/set (0°)
- This setting lights the earth as though by a point source at the
centre of the sun; the sunrise/set line is shown as though sunlight
ceased when the centre of the sun crossed the horizon, and so no
twilight can be seen; exactly half the globe is lit. The line between
the dark and light sides of the globe is the terminator.
- Ideal sunrise/set (0°50’)
- This is a practical and generally accepted definition of sunrise and
sunset (fifty minutes of angle after the theoretical sunrise/set) which
allows for the diameter of the sun and common atmospheric effects.
The line seen on the globe joins the points at which the disc of the sun
will have just disappeared or be just about to appear, and encloses a
very narrow band of twilight.
- Civil twilight (6°)
- This setting marks the end of ‘civil twilight’ – a
convenient point used for legal purposes (for example, when it is
too dark to carry out certain tasks). Please note that PMGlobe’s
rendering of this line must not be used for any critical or legal
decisions; errors and bugs are always possible, and the definition used
by PMGlobe (six degrees after the theoretical sunrise/set) may not
match the legal definition used in your country.
- Naval twilight (12°)
- This setting indicates the points where it is dark for all practical
purposes: the centre of the sun is twelve degrees below the horizon.
- Astronomical twilight (18°)
-
At this distance from the terminator no effects from sunlight can be
detected, and the centre of the sun is eighteen degrees below the
horizon.
- Background colour
-
displays a sub-menu which lets you select the colour of the background
used for the globe (the area of the PMGlobe window outside the circle of
the globe image).