PMGlobe, version 3.31 3 Aug 2009
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Date and time limits and accuracy

The data used for presenting the globe are loaded in compressed form (from about 22kB for the Classic map, and 300kB total for the default images). The coastline and lake data for the Classic map were compiled from several different sources, with some manual editing to improve the representation in critical areas such as narrow straits and isthmuses. The accuracy of the coastline and other data is not guaranteed in any way, but is believed to be within 11km (7 miles) in the worst case (near the equator) and significantly better in the East-West direction in Northern and Southern latitudes.

When distance calculations are carried out, a mouse selection can only be determined to the nearest pixel (picture element) on the screen. The longitude and latitude are then reported as at the centre of that pixel. Distances are then calculated between the two coordinates selected and should be accurate to the nearest unit (km or mile), or 0.2%, whichever is greater. The 0.2% limit is a consequence of the assumption used in the calculation that the earth is spherical (which it is not).

Time and sun position calculations are only important when the sunlight option is selected. In this case, PMGlobe needs to know the current date and time-of-day (taken from your computer’s clock and time zone settings – make sure these are set correctly). PMGlobe uses Windows’ time zone mechanism to allow for daylight savings time changes and so on; these are normally automatic.

From the current time and time zone information, PMGlobe can determine the apparent solar time (which is as much as 16 minutes different from the Civil time used for clocks) and hence the sun’s position. This is then used to display the globe as though lit by the sun: the light/dark dividing line thus shows where the sun is rising or setting.

The various calculations done should give an accuracy of sun position that leads to a sunset or sunrise indication that is correct to within twenty seconds of time. Actual sunset or sunrise times will be a little different because of atmospheric effects, which vary with the time of year, the weather, and latitude (in very Northern or Southern latitudes, especially, scattering effects can greatly prolong daylight in summertime). However, PMGlobe does give a useful indication of sunrise and sunset, and of course lets you see at a glance which parts of the globe are in night or daylight.

Celestial events such as equinoxes, and solstices especially, are very dependent on the earth’s movements; equinoxes should be correct within some tens of minutes, solstices should be correct within about ten hours.

Please note that results will be incorrect if you set your computer’s date to be earlier than 1 January 1990. Also, the formulae used to calculate the sun’s position may prove to be inaccurate at some distant time in the future, because the earth’s movement is not entirely predictable.

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Copyright (c) IBM Corporation, 2009. All rights reserved. ©
Author: Mike Cowlishaw,