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For starting Shabbat, is sunset astronomical or visible?
A few years ago, on the fall equinox, I noticed that the day was longer than 12 hours according to the (secular) sunrise/sunset times. Curious, I did some investigation. One reason for the difference is how we measure -- the equinox is based on when the center of the sun is visible while sunrise/sunset times are based on when the top edge is visible. That only accounts for a couple minutes; though; the bigger factor is atmospheric refraction (discussed in simpler terms here), which, in a nutshell, means that the sun is visible to us in the sky even when it is below the horizon, for several minutes.
This made me wonder about the effect on halachic times. This article says that the halacha is that Shabbat begins at sunset, though we add time for safety. I know that, l'hatchila, we add (usually) 18 minutes before the start of Shabbat or Yom Tov to provide a cushion. That's more than enough to account for any false effects due to refraction, but I've also been told that, b'dieved, if one is running late and has to cut into those 18 minutes, it's still ok so long as you actually began Shabbat/Yom Tov before the end of those 18 minutes, at sunset. And therein lies my question.
Regardless of how we halachically define sunset (that is, what part of the sun we're comparing to the horizon), we might see the reference point above the horizon and conclude that it is not yet sunset when the sun has actually already set. Does this mean that the day has in fact started -- we go by where the sun is, not where it appears to be -- and if you light at the 15th of the 18 minutes you're kindling fire on Shabbat? Or, halachically, is sunset based on what we see, not what we now know happens scientifically? According to the abstract of this paper, western science knew about atmospheric refraction in the second century BCE; it's not new.
I know there's discussion (somewhere) of Shabbat times if you're in the mountains or have an obstructed view of the horizon, and maybe that's relevant here. That's a more localized situation, while atmospheric refraction happens everywhere all the time, so I don't know if that makes it a different case. I know that it also takes eight minutes or so for sunlight to reach earth at all, but that feels like a different problem.
Does Shabbat really start sometime during the 18 minutes?
I asked this question some years ago elsewhere and didn't get an answer. I came across a blog post I'd written about it at the time, and that prompted me to re-ask it here.
1 answer
TL;DR Shabbos starts at the generally accepted astronomical definition of sunset, and the 18 minutes is before that time.
There are a few separate issues here:
Halachically, when is "Shkias Hachamah" == Sunset?
Sunset is actually a relatively universally agreed upon time. A quick look at MyZmanim. From Wikipedia:
The time of sunset is defined in astronomy as the moment when the upper limb of the Sun disappears below the horizon
That can be calculated for any day and any location in the world using well-established formulas.
Halachically, it doesn't matter if you can see it on a given day due to weather. There are opinions, however, that sunset (and sunrise and related times) may vary depending on elevation above sea level. For example, for US zip code 20902 for Friday, December 3, the normal time according to MyZmanim is 4:27:42 EST. Changing elevation to 100 meters (close enough, and a nice round number) changes the time to 4:27:39 - not much of a difference, and the typical published time is "round down to the minute" - i.e., 4:27 either way. In higher elevations it can have a real effect.
In fact, when I wrote an Eruv notification system many years ago, my main contact said to list candle lighting (18 minutes before sunset) as pretty much everyone agrees on that, but not to list the end of Shabbos as there are many different opinions, as explained below.
The "18 minutes"
Shabbos does not start until sunset (however that is defined). The 18 minutes is a buffer. Most places in the diaspora agree on 18 minutes. There is a nice article from Chabad which explains some of the reasons behind it. But everyone agrees this 18 minutes is before Shabbos actually begins. In Jerusalem, and many other parts of Israel, candle lighting is normally earlier - e.g., 40 minutes before sunset.
"Bein Hashmashos" == literally "between the suns" - The time between Shkias Hachamah and Tzeis Hacochavim (3 stars = end of Shabbos).
The actual start of Shabbos (or rather, the start/end of any Halachic day) is somewhere between sunset and "Tzeis Hacochavim". Tzeis Hacochavim has many different definitions. The general concept is "dark enough to see stars", but there are many ways to define this, which includes variations based on latitude, day of the year and other factors. In particular, Tzeis Hacochavim (and similar times in the morning before sunrise) can be based on a set time or "minutes as degrees" - taking minutes of time and using those to determine a relative astronomical phenomenon defined in degrees and then calculating that astronomical phenomenon for a particular time/place. The end result is a lot of different times for Tzeis Hacochavim. The two I am most familiar with are "42 minutes" (which is probably the most commonly used in the diaspora) and "72 minutes".
The key point though is that we don't know exactly when in the range of Shkia to Tzeis is the actual start of Shabbos. Due to the importance of observing Shabbos (and Yom Tov and fasting on Yom Kippur), we treat the entire time between Shkia and Tzeis (Bein Hashmashos) at the start and end of Shabbos as if it is all Shabbos, even though it can't possibly all be Shabbos on both ends. There are some Halachic ramifications of this - e.g., I believe in certain cases Bein Hashmashos has some leniencies with respect to asking a non-Jew to do certain activities for a Jew.
And if you are at the South Pole or on the International Space Station, ask your LOR.
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