AstronomyEquinox sun above horizon all day, at the poles

Equinox sun above horizon all day, at the poles

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The video above is from the South Pole throughout polar sundown in March 2022, when the sun goes from having been up for six months to being down for the subsequent 6 months. So, throughout the equinox, the sun scrapes alongside the horizon throughout all the day. However do you know that the equinox sun appears to be like precisely the identical at each poles on the similar time? Video through South Pole Telescope.

An EarthSky reader despatched us a query awhile again – on the September equinox 2023 to be actual – to ask how an equinox appears to be like on the North and South Poles. Would it not, because of refraction, be above the horizon at each poles on the similar time, and all day lengthy? This become a pleasant change of emails (we do love chatting with our readers!) the place we mused on the subject of polar equinox. Our reader, Rod Cavanaugh, had all of it found out! He needed to ensure his concepts had been right … and they also had been.

Equinox sun on the North Pole

On the poles, the yr will be divided into one night time and one day, or reasonably, half a yr of darkness and half a yr of daylight. The transition between these two states of sun-presence on the sky happens throughout the equinoxes. On the North Pole, the sun has now been beneath the horizon for all the winter. Through the March equinox, it pops above the horizon – polar dawn – to then sit on the sky for all the summer time, increased and better every day till we hit the summer time solstice. So, throughout equinox day, it follows the horizon all day lengthy.

Polar dawn for South Pole in September, sundown in March

On the South Pole, it’s the reverse: the sun has been above the horizon for half a yr, and now, throughout the March equinox, the day transitions through polar sundown, into everlasting (or, reasonably, half a yr of) darkness.
So, half the sun is beneath the horizon and half is above it, at each poles. Or no less than that’s how it could appear to be with out the Earth’s environment.

Concordia station in Antarctica isn’t positioned on the South Pole, however close to it. So that they expertise dawn in August, not September, and have 4 months of darkness, not 6. Right here, researchers have fun the dawn at Concordia analysis station on August 11, 2020. Read more about this image!, which is through ESA.

The sun is above the horizon at each poles on the similar time

However, right here comes the shock! Because of refraction, a sun on the horizon doesn’t look like half beneath it and half above it, despite the fact that it geometrically could be. The environment refracts the sunshine that hits your eye, such that the sun seems to be about half a level above the horizon at each locations. Our reader, Rod Cavanaugh, wrote:

If the environment’s refraction raises the sun about one-half a level and the sun’s angular diameter is about one-half a level, then at sundown the precise/geometric sun ought to be fully set on the time the obvious/refracted sun first touches the horizon.

However from the poles the middle of the sun ought to be precisely on the horizon on the equinox, so there ought to be a niche of one-quarter a level between the obvious sun and the horizon, that means the sun is totally above the horizon at each poles on the equinox.

Sketch with a curved earth section to the right, a straight green and a curved yellow line, and 2 yellow dots depicting the refracted and unrefracted sun to the left.
When the sun is just under the horizon (S), refraction lifts it to seem simply on prime of it (S’). The quantity of refraction close to the horizon is round 0.5 levels, which additionally occurs to be the sun’s obvious diameter. Picture through britastro.org.

Not solely on the horizon, however above it

Rod requested if this seemingly counter-intuitive state of affairs is right. And certainly, it’s! A sun that sits half a level (30 arcminutes) beneath the horizon is refracted 33 arcminutes. However the equinox sun is a bit increased (not beneath the horizon but, however half above and half beneath). Which means there shall be a niche between horizon and sun. The hole gained’t be precisely 0.25 levels, however the prime shall be refracted between 25 and 29 (we are able to assume 27) arcminutes, and the underside half (0.25 levels beneath horizon), about 31 arcminutes. So, Rod’s thought experiment is right. There shall be a niche of a few quarter of a level (about 15 arcminutes), so the sun will seem a bit above the horizon at each places. Additionally, the sun will seem oval as a result of the higher limb is refracted lower than the decrease.

Learn extra:

Atmospheric refraction explained

Watch the sunset at the South Pole on NOAA/ESRL’s South Pole Live Camera

Backside line: We all know the equinox sun rises due east and units due west. However what occurs at Earth’s poles? Seems the North Pole sun is above the horizon all day lengthy. And it appears to be like precisely the identical on the South Pole!



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