If the Earth rotates on its own axis, then what do we mean by sunrise and sunset?
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3 Answers
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Yes the earth rotates around its axis, which gives rise to the motion of the sun as we see it.
To understand sun rise you'd have to start with a reference frame. For us, the earth is considered to be the inertial frame for any apparent motion in the celestial sphere, it simplifies the computations. Thus any motion of celestial bodies is defined with respect to that frame.
So in that context Sunrise would be the time when for a given latitude, when the sun first appears above the local horizon, and sun set would become the time when the sun disappears below that local horizon.
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$\begingroup$ What definition are you using? Wikipedia says it's as soon as a bit of sun peeks over the horizon $\endgroup$– ignacioCommented Jan 17, 2016 at 12:42
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$\begingroup$ you are right I was taking the websters definition 'the time when the sun appears above the horizon in the morning' too literally. I'll modify it $\endgroup$ Commented Jan 17, 2016 at 13:19
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Sunrise is the moment when the point you are located on, on Earth, gets to receive its part of the sunlight (due to the Earth's rotation you spoke about). And sunset is the moment when the point you are located on, on Earth, has made its turn and faces away from the Sun, then preventing you from receiving its light.
When the Earth will have made another half of its rotation, you will get to see another sunrise.
The Earth makes a full rotation in a 24h cycle. This is why you get a sunrise and a sunset everyday :)
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2$\begingroup$ and the technical definition is: Sunrise is the moment the first direct rays of sunlight reach you. Sunset is the moment the last rays of sunlight fail to reach you. So on the Equinoxes "daylight" is actually longer than "night" by several minutes. They'd be equal if Sunrise and Sunset were measured at the point the Sun was "half set" and "half risen". $\endgroup$– Jim2BCommented Jan 12, 2016 at 20:22
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$\begingroup$ Thank you, @Jim2B. Is it true that there's another element in the difference in the lengths of day and night (even at an Equinox)? Namely that atmospheric diffraction means that when, for example, we see the sun's disk balanced on the horizon at sunset, the sun has actually already set? $\endgroup$ Commented Jan 18, 2016 at 20:53
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$\begingroup$ Yes :) I forgot that diffraction affects the timing too. $\endgroup$– Jim2BCommented Jan 18, 2016 at 21:44
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$\begingroup$ it's also not exactly or only bound to the rotation period of Earth. Think of places where you are North or South wrt the polar circles and eternal night or day during solistices. $\endgroup$ Commented Oct 3, 2021 at 7:46
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Its simply mean ,when earth rotates (as Sun don't move) ,the instant when the light of sun falls on the earth called sunrise and its just opposite mean sunset.
