Mark Himelfarb wrote:

Dear Rabbi,

We light our Shabbat candles at least 18 minutes before the official sundown listed for our locale. Is there any significance for 18 minutes? If Shabbat begins at sundown, why should I light my candles 18 minutes before sundown and not just one minute before?

Dear Mark,

The Shulchan Aruch states that one should "add from the weekday onto Shabbat." (This idea is derived from a verse in the Torah.) This is called "Tosefet Shabbat" - an addition to Shabbat. The Shulchan Aruch suggests a variety of possibilities for how much time to add. It seems that the custom of 18 minutes arises from the Halachic time measure called a "Hiluch Mil," (pronounced he-looch meel), which means the time it takes to walk a Mil. The word "Mil" is derived from the Roman "Miliarium" which was 1000 paces (approximately 3000 feet). According to the ancient American commercial this would be the amount one would walk for a Camel. The Halachic Mil and the Roman Miliarium differ only slightly in length. The time it takes to walk a Mil is 18 minutes.

Although the predominant custom is to light Shabbat candles 18 minutes before sundown, there are other customs. For example, the custom in Jerusalem and in Petach Tikvah is to light candles 40 minutes before sundown. Wherever you find yourself for Shabbat you should light according to the custom of that place.

Sources:

• Shulchan Aruch - Orach Chaim, 261:1-4.
• Otzar Yisrael Encyclopedia, entry 'Parsah.'