10

The orbit of Earth is elliptical and the nearest distance to sun is perihelion position. Should it not be the shortest day in the northern hemisphere?

T.J.L.
  • 103
  • 2
Mohan Mone
  • 941
  • 1
  • 7
  • 12
  • 14
    Why should it be any different if you apply the exact same logic to the Southern Hemisphere? North and South aren't related to perihelion/aphelion. – Eric Duminil Jan 12 '20 at 16:46
  • 13
    The real question you should be asking is why perihelion and the northern winter solstice are so close together, only a couple of weeks apart. And the answer is that it’s just a coincidence. – Mike Scott Jan 12 '20 at 19:09
  • 1
    Imagine if they are the same. Then why are Perihelion and Shortest day in South Hemisphere different? – user253751 Jan 13 '20 at 17:31
  • 1
    Clearly, the Illuminati got their math wrong when they were coming up with the Globe Earth story. – Acccumulation Jan 14 '20 at 06:26
  • 2
    The shortest day (in the northern hemisphere) occurs when Earth's axis of rotation is tilted furthest away from the sun.

    Perihelion occurs when the orbit of Earth takes it nearest to the sun.

    The events arise from unrelated causes and are therefore also unrelated.

     – Vince O'Sullivan Jan 14 '20 at 15:03
  • I asked a very similar question 6 months ago: https://astronomy.stackexchange.com/questions/32470/aphelion-and-the-solstice – badjohn Jan 17 '20 at 16:49

3 Answers3

32

We have "days" because the Earth is rotating. The shape of the orbit has little effect on the daily cycle of light and dark.

The varying length of the day is determined by the tilt of Earth relative to the plane of the orbit of the Earth around the sun. When the Northern hemisphere is pointed away from the sun, then the sun is low in the sky and is below the horizon for longer. So the shortest day happens when the Earth is at the point in its orbit when the angle that the Earth's axis makes with the line from the Earth to the Sun is greatest. This is on about the 21 December.

The longest and shortest days of the year occur because of the tilt of the planet, which also controls when the Equinoxes and Solstices occur. The tilt of the planet has absolutely nothing to do with its location in orbit, and it's only pure coincidence that they're so close together. Also, perihelion moves over time (apsidal precession) and in 56,000 years it would occur on the longest day in the Northern Hemisphere. Except for the fact that the Earth's axis will have also precessed (with a period of around 26,000 years) so the shortest day will occur somewhere else on in the Earth's orbit

For the sake of clarity: The equinox does not occur on the minor axis. The solstice does not occur on the major axis. The tilt of the Earth to then orbital plane does not change over the course of the year, and the Earth's rotational axis is no aligned with the major axis of the Earth's orbit.

Michael MacAskill
  • 109
  • 4
James K
  • 120,702
  • 5
  • 298
  • 423
  • 1
    +1 Indeed. Also, perihelion moves over time (apsidal precession) and in 56,000 years it would occur on the longest day in the Northern Hemisphere. Except for the fact that the Earth's axis will have also precessed (with a period of around 26,000 years) so the shortest day will occur somewhere else on in the Earth's orbit – Dave Gremlin Jan 12 '20 at 16:56
  • Let me put up the question differently. Its really not Northern or Southern hemisphere. The question is Should the Equinox and Solstice occur at Minor axis and Major axis since the orbit is Elliptical? – Mohan Mone Jan 12 '20 at 23:18
  • 2
    @MohanMone The answer remains exactly the same. The longest and shortest days of the year occur because of the tilt of the planet, which also controls when the Equinoxes and Solstices occur. The tilt of the planet has absolutely nothing to do with its location in orbit, and it's only pure coincidence that they're so close together. – HiddenWindshield Jan 12 '20 at 23:55
  • At equinox there is equal day and night. This means the tilt towards sun is ZERO, hence the point should be on Minor axis otherwise any other point on elliptical orbit either North or South pole will be towards the sun-may be extremely small angle-and hence day and night can not be exactly equal. Hence my question- if the equinox is on minor axis then solstice should be on major axis – Mohan Mone Jan 13 '20 at 01:16
  • 5
    @MohanMone The piece you are missing is the tilt of the earth is not parallel to the major axis of the elliptical orbit, so the equinoxes/solstices are not lined up with the major/minor axes. There is no "the tilt towards sun is ZERO, hence the point should be on the Minor axis" because there's no reason for it to be. – throx Jan 13 '20 at 05:27
  • I feel that this could be easily demonstrated by a nice 3D animation (that exaggerates the eccentricity of the orbit), but it seems like quite a lot of work for just a SE answer. But perhaps such an animation already exists? – JiK Jan 13 '20 at 13:20
  • 1
    It is worth adding that this coincidence is only valid for the current timeframe. The Earth axis changes because of the precession movement but with a period of 26k years. http://hosting.astro.cornell.edu/academics/courses/astro201/earth_precess.htm but @DaveGremlin just did. OOps. – Mindwin Remember Monica Jan 13 '20 at 15:33
15

Equinoxes and solstices are determined by the direction of tilt of the planet's daily rotation. The direction of tilt of a planet's rotational axis is essentially random, (mostly) constant, and unrelated to any property of its orbit.

I did not calculate the precise correct position of the sun in the below diagram, but it doesn't need to be precise to illustrate the point. The planet's tilt is toward the upper right of the diagram at all points in the orbit, changing only very slowly in a process called precession that takes many millennia.

Diagram of equinoxes and solstices on an orbit

Douglas
  • 251
  • 1
  • 3
  • 1
    Nice diagram. Just to make it clear that there's no preferred direction or no preferred hemisphere, "Tilt" could have arrows in both directions and "Summer Solstice" and "Winter Solstice" could just be "Solstice"s. – Eric Duminil Jan 13 '20 at 14:23
-1

The amount of daylight does not define a day. A day is defined as between noon one day until noon the following day. Because the earth is closer to the Sun during the Perihelion, it moves faster through the orbit. This means that it travels farther between noon Jan 4 and Jan 5 than between July 4 and July 5. Because the earth moves farther, it must rotate more before it is noon the following day. That makes January 4 a few seconds longer than July 4.

Mark Daniel
  • 1
  • Your answer is technically correct, but the original question is using "length of day" to mean the length of daylight: the time from sunrise until sunset. That occurs on the winter solstice (Dec 21 for Norther hemisphere), not at perihelion. – JohnHoltz Dec 21 '23 at 17:52
  • The length of a day remains at approximately 24 hours all year long and the length of a day is the same in the northern and southern hemisphere. The words "day" and "daylight" are the cause of the confusion. – Mark Daniel Dec 22 '23 at 19:24
  • Unfortunately, "day" is ambiguous in English. It can mean "a period of 24 hours" but it can also mean "the period from sunrise to sunset". You just have to guess from the context. Some languages have separate terms. – badjohn Dec 26 '23 at 10:14
  • The solar day length varies from 24 hours - ~22 seconds to 24 hours + ~30 seconds. See my answer https://physics.stackexchange.com/a/790695/123208 – PM 2Ring Dec 26 '23 at 11:31