One Day = 24 Hours ? Learn Astronomy

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Day = 24 Hours? For practical purpose, we follow the solar time. But earth takes less than 24 hours to rotate itself. The earth rotates about its axis (diameter) from west to east in 23 hours 56 minutes 4 seconds of apparent solar time.This period is called a sidereal day. Due to this rotation, the observer is carried eastward constantly and all the celestial objects including Sun, moon, stars appear to rotate from east to west. Why this period is less than 24 hours? Apart from this diurnal motion, earth describes an elliptic orbit around the sun. Due to this motion, with respect to the position of a star, earth takes less than 24 hours for rotation.

Where Sun rises ? In the East direction. Correct! But is it exactly East? No! Sun rises at east exactly and sets at west exactly only on two days namely, March 21st and September 23rd. These days sun traces a semicircle on the sky. That is the great circle called celestial equator of your place of observation. These days the duration of daytime is 12 hours and the durations of day and night are equal.

Each day sun travels in a circular path which is parallel to your equator. After March 21st, the sun gets northwards, its daily path getting shifted away from the equator, moving the position of rising on the horizon northward and the duration of day time increases above 12 hours and night shrinks below 12 hours. This continues till the position of rising is about 23^o 27^' north of east on June 21st. This is the longest day for the region (shortest day for south torrid/temperate zone).

After June 21st, the daily path of sun is getting shifted towards the equator. On September 23rd, the sun describes the equator, rising at the east point and setting at the west point (thus day = night). Thereafter the sun gets southwards, moving the position of rising on the horizon southward and day time decreases and night increases. This continues till the position of rising is about 23^o 27^' south of east on December 22nd. This is the shortest day for the region (longest day for south torrid/temperate zone).

After December 22nd, the sun moves towards the equator, duration of night decreases and that of day time increases till they become equal on the next March 21st. You can not watch this exciting movement in a place on the arctic circle.