Why does the Sun always rise in the East?

Question

Earth is rotating on its own axis & also revolves around the Sun, then how come the Sun always rises in the East?

score 31 · Answer 1 · edited Jun 04 '20 at 16:03

Whether the sun "rises" in the east depends on your position on earth, and the time of the year. In northern latitudes, during the summer, the sun rises significantly North of East, and in the winter it rises in the South. For example, today's sunrise/sunset directions in Umeå Sweden, look like this (source: www.suncalc.net)

Map, centered on Umeå, Sweden, showing sunrise/sunset directions

The yellow line shows the direction of sunrise, the orange line the current direction of the sun, and the red line the direction at sunset.

As you can see, the sun never gets close to being in the East...

The reason for this is the fact that the earth's axis of rotation is not perpendicular to its plane of rotation about the sun (the ecliptic). The same mechanism that causes summer and winter in the higher latitudes gives rise to this changing direction.

But as for the fundamental question: the rotation of the earth about its axis is much faster than the rotation of the earth about the sun - so the rotation of the earth is dominating the direction of the sunrise. Now if the earth stopped rotating altogether, the sunrise would be in the "Westerly direction", since the direction of both rotations (seen from say the North Star) is in the counterclockwise direction. It would just rise and set only once a year... But the year is 365.24 days, and while that is so, the sun will rise "mostly in the east".

A small addendum: because the earth's orbit is elliptical, its angular speed relative to the sun changes a little bit with the seasons. This is enough to make a sundial "off" by up to 15 minutes, depending on the time of year. This is captured in the "equation of time" and shown, for example, in this graph (from http://upload.wikimedia.org/wikipedia/commons/0/02/Tijdvereffening-equation_of_time-en.jpg):

enter image description here

And just for your amusement - on October 25th, the sun briefly rises in the South on Svalbard (Spitsbergen) before disappearing for the winter...

I dunno, I think this is being rather pedantic. I would take "in the East" in this context to mean anywhere in the set of directions that are closer to east than west. — David Z, Nov 25 '14 at 08:57
It's unclear whether the OP, or others who have the same question, actually know that the Sun doesn't rise due East. For that reason I think the answer is useful. — BMS, Nov 25 '14 at 14:08
@BMS OK, I see what you mean, although I personally wouldn't make that judgment without more evidence of it being the case (such as the question being "why doesn't the sun rise in the east"). — David Z, Nov 25 '14 at 16:13
Floris, if you didn’t read the comment wiped out by David Z, let you know that Ī respect your answer. — Incnis Mrsi, Nov 25 '14 at 16:28
@DavidZ - note that I cover both the "not quite in the East" part, and the "because the earth's rotation dominates" part. It seems to me both are relevant - I'm sorry you think it pedantic. "pedant noun a person who is excessively concerned with minor details [...]". If you have ever navigated a ship by the celestial bodies, the direction in which is the sun rises is NOT a "minor detail". — Floris, Nov 25 '14 at 21:02
I doubt that the OP has ever navigated using the celestial bodies ;-) But anyway, it's not bad. — David Z, Nov 26 '14 at 04:01

score 13 · Answer 2 · 2014-11-26T07:11:51.573

The Sun does not rise, it is the horizon that goes down.

You say that Sun rises in the East (with a certain degree of oscillations due to the tilt of the axis) just because the Earth spins from West to East. The revolution affects the difference between sidereal time and solar time, and makes the solar day $\approx 4$ minutes longer

If the Earth spinned in the opposite direction the Sun would appear to rise from the West.

It does not address the question in any meaningful way. First of all, the answer ignores the problem “how west–east direction is defined”? - Incnis mrsi

Why should my post answer that off-topic and silly question? Incnis, you ask a new question about that and I will address it.

Any definition is a convention, you can debate for hours which is the best for Venus. OP didn't ask about that, he knows the conventional definition and we all know what 'East' means.

Some posts introduced foreign elements or useless nitpick in order to differentiate themselves, I just gave the basic answer.

It does not address the question in any meaningful way. First of all, the answer ignores the problem “how west–east direction is defined”? — Incnis Mrsi, Nov 25 '14 at 14:54
@IncnisMrsi This answer is just fine. The so-called 'problem' (why is this a problem?) of "how is west->east defined?" is actually written as part of this answer: it is defined that way based on the Earth's rotation. — Lightness Races in Orbit, Nov 25 '14 at 15:15
You should respond to comments with a comment. The rebuttal is not part of your answer to the question and should not be included as such — Jim, Nov 26 '14 at 14:34

score 6 · Answer 3 · 2014-11-25T07:07:46.130

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It takes a year for Earth to revolve around the Sun, and only one day to rotate about itself. That is why you can, for most practical purposes, forget about the revolution (which causes the different seasons) and concentrate only on the rotation, at least fo sun raising purposes.

edited Nov 25 '14 at 07:07

answered Nov 25 '14 at 06:50

@Alba you contradict yourself: "and makes the solar day ≈4 minutes shorter" – Nov 25 '14 at 08:09
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I deleted an inappropriate comment. Remember, any time you see inappropriate behavior, just flag it for moderator attention. – David Z Nov 25 '14 at 08:58

user22180 · Answer 4 · 2014-11-25T09:35:21.207

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At first ask yourself the question what do you mean by "East"?

Other answers have already said that why you should only concentrate on earth spin rather than its orbital motion towards that sun.

See North, East, West and South are not absolute directions and changes with the latitude and longitude. So wherever you are on the earth surface, depending on your local position you have a fixed North, East, West and South directions with respect to your own co-ordinate system. But these directions which are fixed with respect to you are changing with respect to an observer at the sun because of the earth's spin. Now as the earth is rotating counter-clockwise(as seen from the top of the North pole) you will always encounter rising sun in the direction East irrespective of which month it is. Try to visualize and you will get it.

So if in the planets Venus and Uranus we do define "North, East, West and South" in the same way as in the picture in the link, there sun rises in the West due to their Retrograde motion.

edited Nov 25 '14 at 09:35

answered Nov 25 '14 at 09:28

user22180

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The only way I would think a compass would rise in the west of a celestial body would be if were rotating prograde, but at a rate of less than one rotation per orbit. If a planet were rotating retrograde, then its "north" would point the opposite direction from prograde planets' "north", and its "east" would face the sunrise as with prograde planets. – supercat Nov 25 '14 at 18:35
@supercat, yeah I agree with you as I have already confessed in earlier comments that if that is the way North is defined there then the sun rises in the east in those planets too. – user22180 Nov 25 '14 at 20:00
@user22180: Did you notice, though, that I stated how a planet could have a western sunrise? – supercat Nov 25 '14 at 20:02
@supercat, Yeah I got that too, and that would be due to very fast orbital motion, which is not the case here. – user22180 Nov 25 '14 at 20:08
@user22180: More likely really long "days", since fast orbital motion would imply that the body was very close to the sun. – supercat Nov 25 '14 at 20:12
@supercat, yeah you are right, very slow spin could also be a possibility. – user22180 Nov 25 '14 at 20:13

score 2 · Answer 5 · answered Nov 25 '14 at 09:31

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The question is quite geocentric, as it is dependent on the observer's location.

On Venus, which has a retrograde axis of rotation, the Sun appears to rise in the West.
In Arctic regions, the Sun may not rise or set for up to six months of the year, and even then, it may not be clear in exactly which direction the sunrise/sunset occurs.
Other suns in the sky (ie stars) rise and set at all points around the compass.

answered Nov 25 '14 at 09:31

iantresman

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Again, what is West on Venus? Why are you so sure that know what it is? – Incnis Mrsi Nov 25 '14 at 14:50
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@IncnisMrsi: We decide what it is. West and East are well-defined cardinal compass points. What else would they be? – Lightness Races in Orbit Nov 25 '14 at 15:15
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@LightnessRacesinOrbit: For a rotating planet, I would think it most logical to define the poles as being the points where the surface intersects the axis of rotation and specify east as the direction in which any point on the surface of the body is moving relative to its center of mass. North would then be the direction toward the pole which would appear to the left of an east-facing observer. Such a definition would be independent of the existence of other celestial bodies. One could also define east as a direction relative to celestial north, but that seems less logical. – supercat Nov 25 '14 at 18:26
@LightnessRacesinOrbit: Among other things, the only time directions defined relative to a body's rotation would be ambiguous would be if the body had no regular rotation--a very odd situation given that even tide-locked bodies rotate once per orbit. Directions for "east" and "west" based on celestial-north, however, would have no consistent meaning if a planet's axis were orthogonal to the celestial north/south axis. – supercat Nov 25 '14 at 18:31
@supercat Given that the cardinal directions are defined for use with navigation by magnetic fields, that is the basis on any other world for how to define them. The north and south poles should be defined such that the north pole of a compass points to the defined north pole of a planet. The remaining directions are defined according to that. For planets without a magnetic field, there is no reason to have specific definitions for the cardinal directions except to be consistent with Earth's definition. – Jim Nov 25 '14 at 20:00
The North pole of Venus, since navigation of its surface is not required, is defined according to its axis of rotation. But its north pole is closely aligned to ours, not our south pole as your system would have it. If we travelled to another stellar system and colonized a planet with no mag field. North, east, west, south would be useless except for orienting maps. We could put east and west along the axis of rotation, but out of homesickness, we'd probably arrange it so that the Sun rose in the east – Jim Nov 25 '14 at 20:05
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@Jim: I think people probably had vague concepts of east and west before magnetism was discovered, though seasonal variations in the sun's position may have been confusing. Compasses were useful because even at night they could indicate a direction which was generally 90 degrees to the left of the direction from which the Sun would rise, but I don't know that their behavior ever "defined" north. – supercat Nov 25 '14 at 20:08
@supercat you're right, I was thinking of the reverse. That North defined the naming conventions of magnetic poles because the north pole of a magnet points to the geographic north pole. East was defined by the direction of sunrise. North was defined by Polaris and meteorological phenomena. But the North pole of venus, is defined in the same basic direction as ours. The Sun rises in the west on Venus – Jim Nov 25 '14 at 20:41

score 0 · Answer 6 · edited Nov 27 '14 at 10:40

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It looks like the Sun rises in east and set in west, but actually it is altogether vice versa. Our earth rotates around the sun in west to east fashion. Since we see ourselves as stationary in our frame of reference(i.e. earth), the sun seems to rise and set in east to west fashion. Similarly we have winter and summer as seasons due to the revolution of earth around the sun.

edited Nov 27 '14 at 10:40

John Rennie

355,118

answered Nov 25 '14 at 08:11

Android7

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Apogee and perigee (or apohelion and perihelion in the case of rotation about the sun) don't affect summer/winter much: it's mostly the obliquity (angle between earth's axis and ecliptic) that affects seasons, with eccentricity of the orbit being a secondary effect that will make seasons more extreme when the two reinforce each other. – Floris Nov 25 '14 at 08:18
What do you mean it is vice versa? As Earth rotates, the surface enters the Sunlit zone from east to west, which means the Sun rises in the East. There is no vice versa with appearances being opposite of what is happening – Jim Nov 25 '14 at 19:54

score 0 · Answer 7 · answered Nov 25 '14 at 19:29

Earth is rotating on its own axis & also revolves around the Sun, then how come the Sun always rises in the East?

Earth orbits around the Sun, but the motion can be expressed equivalently in Earth reference frame as Sun orbiting Earth. The orbital speed is not perfectly uniform, but close enough not to be distinguishable in normal life.

Earth also rotates. The plane of rotation is tilted (about 23.5°) compared to orbital plane, but let's ignore that for a moment. The rotation speed is even closer to uniform than orbital speed.

Orbit around rotating body can be expressed in the rotating reference frame, that is the reference frame of you as observer on Earth, as simple orbit with angular speed equal to sum of the two angular speeds. Since both angular speeds are almost uniform, so is the resulting orbit in your reference frame. Therefore you see the Sun move across the sky in always the same direction and always the same angular speed.

Now we established the Sun raises always at the same side (in reference frame fixed to place on Earth) we can call that side East and the opposite side West.

The angle between the two rotation axis cause the lateral shift moving the Sun higher above the horizon in summer (which is when your hemisphere is tilted towards the Sun) and lower in winter (which is when your hemisphere is tilted away from the Sun). The yearly variation make sundials be somewhat ahead or behind clock time depending on time of year (up to about 20 minutes).

score -2 · Answer 8 · answered Nov 25 '14 at 15:27

Sidereal day (prograde)

(image credit: Gdr@Wikimedia)

Essential answer, assuming the observer near Earth’s equator, is:

When the Sun rises, the observer is near the centre of the leading Earth hemisphere (upper side of the blue disk in the image), and east points towards the Sun.
At noon, the Sun is above the observer, and east points backward, against the orbital motion.
At sunset, east points away of the Sun, because the planet made a half-revolution with respect to Sun and the observer is now near the centre of the trailing Earth hemisphere (lower side of the blue disk).
At midnight, the Sun is under the observer, and east points forward, along orbital motion.

Year-round change of direction towards Sun in inertial frame (i.e. with respect to far stars) is not very important, assuming a low axial tilt (Earth has about 23°).

This is an overly simplified picture ignoring both observers at high latitudes (such as myself) and axial tilt. For a retrograde rotation leading and trailing hemisphere will swap, but even this doesn’t cause the central body to rise from different sides depending on time of year. Such dependence will happen only in the case of planetary rotation in a very different plane than the orbital plane, such as on Uranus.

I suspect the image is more confusing then helping, actually. — Jan Hudec, Nov 25 '14 at 19:01

Why does the Sun always rise in the East?

8 Answers8