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**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](https://www.myzmanim.com). From [Wikipedia](https://en.wikipedia.org/wiki/Sunset):

> 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](https://www.chabad.org/library/article_cdo/aid/3240769/jewish/Why-Are-Shabbat-Candles-Lit-18-Minutes-Before-Sunset.htm) 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.