Almanac gives you the capability to determine the times of rising, setting, and meridian transit for the sun, the moon, and the planets. It also enables you to calculate the beginning and ending of astronomical, nautical, and civil twilight. The results of the calculations are formatted and written into a text editor. You can perform all common editing functions and save the data as a text file. The formulae used in Almanac assume an ideal horizon where the intersection of the sky with the apparent plane of the earth's surface at the observer is the horizon. For rise/set calculations, the observer has zero elevation so that the horizon is exactly 90 degrees from the local vertical. Refraction of light passing through the earth's atmosphere tends to make objects appear higher in the sky than they actually are. This effect increases as an object's altitude decreases. Almanac assumes average atmospheric conditions in compensating for refraction. Lunar parallax changes the perceived altitude of the moon because the observer is not at the center of the earth. Parallax makes that the moon appears lower in the sky. Almanac compensates for this effect. Parallax for the Sun and planets is small enought to be ignored for this application.