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While researching an answer to this question I stumbled across the term "חצות האמצעי".
What does it mean? Does it refer to halachic midday, i.e. halfway between netz hachama and shkias hachama, or something else? How does it differ from "חצות האמיתי"?

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Were did you find this term? I looked for it but couldn't find it. – rony Nov 20 '11 at 13:38
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@rony: see second section here: chabad-il.org/hit/hit379.htm#6 – Menachem Nov 20 '11 at 17:38

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It refers to "mean noon," as opposed to "solar noon." True solar noon is when the sun crosses the imaginary meridian in the sky between east and west, i.e., when the sun is exactly in the south. The time this occurs varies slightly every day due to a number of factors; the variation is known as the "equation of time." (See here for an excellent explanation of this phenomenon.) The average time at which noon occurs is called the mean noon, and by definition it is at the same time every day. (Theoretically 12:00, but due to uniform time zones, it is usually off by a few minutes. In NYC, for example, it is at 11:56.)

For more information, see here and here in Wikipedia.

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Great answer! Is there a halachic source for this? – HodofHod Nov 20 '11 at 17:53
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There are many sources who say that the computation of 'mystical' times (for lack of a better term), such as tikkun chatzos and "kiddush-avoidance hour," are based on average hours rather than actual solar hours. Also, R' Moshe Feinstein famously held that all zemanim are to be computed based on the average noon. An exhaustive analysis of the topic (with many sources) can be found in this essay: docs.google.com/… – Dave Nov 20 '11 at 18:50
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@Alex - solar noon is certainly not at the same time every day. It varies according to the equation of time mentioned above. That's why sundials need to be used in conjunction with a graph or table of numbers, to adjust for the difference. It's also why the position of the sun in the sky at any given time follows a figure-8 pattern (called the "analemma") over the course of the year. – Dave Nov 20 '11 at 22:53
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So "חצות האמיתי" is solar noon? – HodofHod Nov 21 '11 at 3:33
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HodofHod - Correct. – Dave Nov 21 '11 at 5:40
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