sun set, after all we still have light from the sun, and we should wait until the upper visible sky is as dark as the bottom of the sky?

To understand this phenomenon let’s understand the following. Why is the far out night sky not lit up by the sun? After all although we obviously can’t benefit and receive light so long as we are on the far side of earth, the deep reaches of space visible to us certainly does receive rays? And the answer is; for the sun’s rays to give light they must impact on something. On what does it impact? On our atmosphere. We also need to understand that the globe of our round earth is surrounded by a mass of air which includes the oxygen we breathe 21%, and nitrogen 78% and additional trace gasses. The ball encircling us is not a void but a true substance. Just as an example one can take oxygen or hydrogen and compress it into a liquid (used as rocket fuel). So too, compressed steam is after all water. Gas/air is not nothing but something. This ball of air/gas encircling us is known as the atmosphere and is held bound to the earth by gravity. A helium balloon will float but only for so high, and when it reaches thin air that corresponds to its weight it will settle in right there, until the balloon develops a leak.

Now the atmosphere has most of its gasses in the Troposphere, a circle of about 7 miles, although the atmosphere generally extends way up beyond the stratosphere to what is known as the Karman Line about 100K out. So while we are on earth, we have a ceiling above us. Once the ball of the sun dips below our horizon it still continues to impact upon the atmosphere and generates sunlight in the sky, and therefore at this point we are still under the influence of the sun’s rays, and therefore it can be said the sun has still not set. What happens is, the ground level darkens first, & subsequently the upper skies darken to match ground level- the lower skies.

So technically we have two sunsets. Therefore we have a legal zone known as “Bein Hashmoshes; ‘between the suns’. This second sunset is designated at 72 minutes after sunset.

And when does this occur 13 ½ minute period occur? Until the moment of colloquial sunset (the ball dipping below the horizon) it is obviously day. When the upper sky has darkened it is definitively night. B”hsm (Bein hashmshes/between the suns) is either immediately after sunset or just prior to ‘72’. Many will erroneously attempt to call it night when they see 3 stars, however the GR”A counsels against this unless one is an expert at determining the appropriate stars (M”B 293; 2) (261;3 B”Haloche)

If we count B’hsm immediately after sunset, then we can say it is day until sunset, followed by the twilight zone, and 13 ½ minutes later it is night. This is known as the opinion of the Geonim (Rav Hai and Rav Shrira) (Litvish). However the Rabeinu Tam position is (Chasidic); (Shabbos 35a) we follow the setting of the sun’s rays and night is 72 minutes after sunset, with the final 13 ½ minutes being the B”hsm, whilst previous to that time it is fully day. With this calculation, used by many, one may daven mincha up and until 58 ½ minutes after sunset. However be careful not to contradict yourself with an early Maariv on the same day. The exception to this rule is, when a minyan has prayed Mincha and if we fear we will not be able to all re-gather for Maariv, we may pray Maariv immediately after Mincha. The other exception is on Shabbat eve, where in the interest of making early Shabbat we may pray early Maariv, even if normally one prays a late Mincha. (As an interesting sidebar, Maariv was created by Father Jacob the night he had the ladder dream. This was the night when Hashem caused the sun to set artificially early, a harbinger hinting at the many times when Maariv is prayed before it is technically night. )

However we note a critical caveat. These times are subject to seasonal and latitudinal changes. These times are given during the spring and fall equinox and at the latitudinal aspect of Jerusalem. Therefore as we know the closer to the poles (further from the equator) the longer sunset lasts. Not a longer day just a slower sunset. And so the 72 and 13 ½ minute would expect to be extended. The Gr”a (oh”c 261; 2) who champions the earlier times explains that we must