Today, in march 20th, is among only two days this year once the sun will directly over the equator. (The other will be September 22nd)

Today, the length of the day and also the size of the night are equal – in every place on earth. Just how cool is that!

From tomorrow, the days will certainly be longer than the nights in the northern Hemisphere and they will certainly officially get in spring. This is the ‘Vernal Equinox’. (‘ver’ is Latin for spring).

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Here in the South, it has actually officially to be autumn since March 1st, not withstanding that the true ‘Autumnal Equinox’ is today. (‘autumn’ is currently in latin!) The office of meteorology claims it sticks come calendar months since “it functions nicely because that everybody”.

Contrary come what most civilization believe, the reason that summer is hotter than winter is not since we are closer to the sun in summer. In fact, in the north Hemisphere, the orbit the the planet takes us further from the sun in summer.

Summer is hotter 보다 winter as result of the tilt in the axis that the planet of 23 degrees. If there to be no tilt, every day would certainly be one equinox and also there would certainly be no seasons. Every day would be a bit like today.


These photos native NASA show the amount of sunshine hitting the planet on different days. You can see the spring and also summer equinoxes in the R hand photos. The optimal L is summer in the southerly Hemisphere, and also the bottom together is summer in the northern Hemisphere.


NASA likewise has a time lapse video of the process

Because the this tilt, in summer

– the days space longer, with more hours of sunlight heating the earth.– the sunlight is higher in the sky, an interpretation more sunshine shines top top the earth much more directly.

I think the 2nd explanation squibs the a bit, and also would advantage from a an ext detailed discussion.

(The most viewed short article on this blog is ‘Why is Summer Hotter 보다 Winter’, posted on the (southern hemisphere) vernal equinox in 2011. The remainder of this article is one updated variation of the article.)

Each morning the sun rises in the east and also sets in the west. Around midday it is at it highest suggest in the sky (the zenith) A shadow stick would cast no zero if the sunlight was directly overhead – yet this only happens if friend are located in the tropics (and also then on just two work a year).

The tropics lie in ~ 23.5 levels of every side the the equator, between the Tropic of Cancer and the Tropic of Capricorn. Rockhampton, Longreach and also Alice Springs room close to the Tropic that Capricorn. The Tropic of Cancer passes v India, Saudi and also Mexico – therefore no location in Europe and no place in the USA is tropical.


The Arctic and Antarctic one (at 66 degrees) note the points phibìc (or south) where there will be some days (or nights) that room 24 hrs long.

How ‘overhead’ the sun is makes a distinction to just how ‘strong’ that is – i.e., how much it heats points up. The less overhead the is, the less efficient at heating things up. We notice the distinction in heat between the high-in-the-sky sun at noon and also the low-in-the-sky sunlight at sunset.

These ‘overhead’ factors must be an ext important 보다 the length of day. The days are much much longer in Stockholm (~ Lat 60 degrees N) in summer 보다 they are right here in Alstonweb (~ Lat 30 degrees S) but it is still much warmer here.

There are two factors why being greater in the sky, or more overhead, provides a difference

The Area Effect

(The concentration of irradiate – exactly how spread out it is at any point.)The much more the sun is directly overhead , the less spread out are its rays once they struggle the earth.A torch shone right down in the direction of the floor gives a a circle of light, yet tilt that a little bit sideways and also the light creates a stretched oval shape. The very same amount of power covers a larger area, therefore at any type of one point the irradiate rays are less intense.

The waiting Mass Effect

The an ext the sun is directly overhead, the less distance it needs to pass through the earth atmosphere.The planet is defended by the gases that consist of the atmosphere. The gas molecules protect against the sun’s heat from burning united state up by mirroring some light ray and absorbing others. If the sun is less directly overhead, the rays have to pass through much more of the atmosphere, and also therefore space obstructed by more of the gas molecules.


The earth’s setting is 480 km thick, which method at the Equator the sun’s beam pass v 480 km of air.

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Here in Alstonweb (30°S), due to the fact that the sun’s rays space angled come the atmosphere, the light ray pass through 548 kilometres of atmosphere (480/cos 30°)

At Stockholm (60°N), they need to pass though 960 km (480/cos 60°)

So the sun rays in ~ Stockhom have passed with twice together much setting as the sun’s rays at the equator. They are tired by the time they with Sweden!

Happy Vernal Equinox come our girlfriend in the North, and happy fall Equinox to us.