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In different regions, the sun is directly overhead at different times of the year. When do these event happen?

JeopardyTempest
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1 Answers1

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Having the sun directly overhead can happen only between the Cancer and Capricorn tropics. That is, only the places between 23.5° of latitude north and 23.5° of latitude south.

  • On the Cancer tropic (23.5° latitude north) it will happen once every year, on the day of the northern hemisphere solstice (about June 21st).
  • On the Capricorn tropic (23.5° latitude south) it will happen once every year, on the day of the southern hemisphere solstice (about December 21st).
  • On the equator it happen twice every year. One on each equinox (about March 21st and September 21st).
  • For any other given place between the tropics, it will also happen twice every year. On the days when the Declination of the sun (a coordinate in the sky analogous to latitude on the Earth), matches the latitude of the place. Various formulas to calculate the declination to various precision can be found at Wikipedia.

This figure would help to visualize the situation: sun_earth_dec21

The configuration depicted, will happen on the southern hemisphere summer solstice (about December 21st): The only day of the year when the sun passes exactly overhead on the Tropic of Capricorn.

When it comes to what time of the day it will happen, it won't happen at local noon (see this question), the exact time requires a lengthy calculation (see this), but a good estimate would be:

GMT - TimeZone + Longitude/15

Where GMT is Greenwich Meridian Time (the same than Universal Time for this effects), TimeZone is the time zone you are at. For instance -7 for the Pacific Time Zone in North America. And Longitude, is the longitude of the observer (expressed as a positive value for East longitudes and a negative value for West ones). The error of this estimate can be in the order of 15-20 minutes.

Camilo Rada
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  • Your last paragraph isn't right. The sun is directly overhead every day of the year at the equator. The closer you get to the tropical boundaries, the less number of days per year it will be. – f.thorpe Jan 29 '18 at 06:03
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    @farrenthorpe I'm pretty confident on this answer (I've expanded it a bit to explicitly cover the equator). In the same way that only one parallel of latitude pases by each point on Earth. Only one "parallel" of declination pass by one point in a observer's sky. That's is the case for the zenith (the point exactly overhead). Therefore, for a celestial object to pass by the zenith, its declination must be the same as the declination of the zenith. And the Sun have declination 0° only twice a year, on the equinoxes. – Camilo Rada Jan 29 '18 at 22:20
  • The sun passes directly overhead twice per day at the equator except on the solstices it is once per day. As you move away from the equator there are less days per year that it passes overhead. If you disagree, I can find a reference. – f.thorpe Jan 29 '18 at 23:02
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    @farrenthorpe I do disagree. A reference would be appreciated. – Camilo Rada Jan 29 '18 at 23:05
  • see in the tropics section here: https://en.wikipedia.org/wiki/Daytime – f.thorpe Jan 29 '18 at 23:06
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    @farrenthorpe That doesn't support what you say. It says "The sun always rises NEARLY perpendicular to the horizon" and then "The path of the Sun lies entirely in the northern half of the sky for the period from the March Equinox to the September Equinox and is entirely in the southern half of the sky from the September Equinox to the March Equinox, with the Sun passing directly overhead at noon on the equinoxes.". So it supports that the sun pass directly overhead only on the equinoxes – Camilo Rada Jan 29 '18 at 23:09
  • Sorry for the confusion. What I should've said is that the sun is directly overhead twice per YEAR for all places in the tropics except the edges of the tropics (capricorn/cancer). Not twice per day. The way you've written you answer makes it seem like it only happens once per year. – f.thorpe Jan 29 '18 at 23:11
  • @farrenthorpe The sun is on the zenith in the equator just on the equinoxes - that is, twice a year. I suggest you to take a globe to understand it. – Pere Jan 29 '18 at 23:12
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    @farrenthorpe I've edited the answer to make it more clear. – Camilo Rada Jan 29 '18 at 23:16
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    @Pere please see my last comment which clears up my mixed-up comments and the point I was trying to make to Camilo – f.thorpe Jan 29 '18 at 23:16
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    Always enjoy pointing people to the neat solarcalc website. By using the slider and switching locations/dates anyone reading this can confirm all of the points to Camilo's answer, and also get a clearer estimate of the date of the year of overhead passage (in the tropics). One takehome: indeed the sun only crosses directly overhead 0/1/2 times per year... but it's interesting that the sun does cross the east-west plane twice per day everywhere (near)Poleward of the current declination lat. Hopefully solarcalc can help people get a better picture of how the sun/earth work – JeopardyTempest Jan 30 '18 at 06:15