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A lot of this is just due to rounding. Most sunrise/sunset tables are in whole minutes, so the changes are pretty choppy. But there is an astronomical reason. If the earth's orbit were a perfect circle, we would see the same change in sunrise and sunset every day. For example, on a given day in the fall, sunrise would be 90 seconds later than the day before, and sunset would be 90 seconds earlier. But it's not a circle, it's slightly elliptical, so the speed we move around the sun changes throughout the year. This causes solar time to be a little off from clock time, and by a different amount each day. This is reflected in the analemma, the figure-8 thingy often found on globes. Also try searching on that word, and you will find a bunch of interesting stuff.

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14y ago
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15y ago

The Earth is tilted at an angle of 23 degrees from being vertical (as compared with the orbital plane of the Earth about the Sun). As the Earth revolves around the Sun during the period of a year, the northern hemisphere is more pointed towards the Sun in summer and more pointed away from the Sun in winter. So, in summer, the Sun rises north of east and sets north of west, giving it an earlier rise time and a later set time than during the winter when the Sun rises south of east and sets south of west. Each day, the times of sunrise and sunset change slightly from the day before because of the slight change in where the Sun rises and sets.

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13y ago

The simple answer is that a day isn't always 24 hours long - it is only the average length.

Since the earth rotates and revolves around the sun counter clockwise when viewed from the north pole, you can realize with some thought that it must rotate one more time per year than day/year. 366.24 rotations, 365.24 days in a year. 365/366 x 24 hours in a day means 23 hours 56 minutes and 4 seconds per rotation. The stars in the sky always follow this exact time.

Two things affect length of a day - the tilt and the speed of the earth while it revolves around the sun. The more the earth is tilted towards or away from the sun, the longer the day. Also the faster it's moving around the sun, the longer the day. Since earth is closest to the sun in January, the days are the longest (greatest time for sun to reach same spot in the sky.) In July these two affect cancel out one an other, so it's not as obvious. Read up about Analemmas.

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Q: As the season progresses the time of sunrise and sunset changes but why is the change in time unequal eg sunrise may change by 2 minutes but sunset may change by only 1 minute?
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