A device, method and medium of world clock for knowing the day part and the night part of the world immediately at any time on a world map and easily finding the positions on a world map where the sun, the moon, main planets, main fixed stars, main nebulae, and main star clusters are right above easily. Subprograms for calculating the calendar time, right ascension/declination, and bright/dark border line, painting out the night part, displaying world map data, redrawing the bright/dark border line, indicating the positions where main heavenly bodies come right above, displaying the date and time, etc., are written in the application program ROM for a microcomputer and executed. The day part (3) of a world map (2) is displayed bright while the night part (4) is displayed dark on a display screen (1) connected with the microcomputer. The positions where the sun (6), the moon (7), main planets, main fixed stars, main nebulae and main star clusters come right above are indicated on the world map (2) along with the local times (8) of several main cities in the world.
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