Shabbat begins 18 minutes before sundown on Friday night. In Jerusalem, Shabbat begins 40 minutes before sundown.

Shabbat begins 18 minutes before sundown on Friday night.

Havdalah is 72 minutes after sundown on Saturday night.

So in a given week, Havdalah time is typically 90 minutes after the previous day’s candle-lighting time (18 + 72 = 90). Sometimes there’s an extra minute or two difference, and that’s due to sunset actually differing by a minute or two between Friday and Saturday nights.

Hebcal computes sunset based on an algorithm provided by the U.S. Naval Observatory (USNO). The USNO claims accuracy within 2 minutes except at extreme northern or southern latitudes.

Comparison to local NWS charts for sunrise and sunset (which are cheap and easy to come by) shows that with the double precision calculations, the charts produced by this program are no more than 1 minute removed from those charts in lattitudes lower than 41 degrees. Candle lighting times agree with those on popular calendars also to the 1 minute accuracy.

That said, the USNO algorithm can only approximate the candle-lighting times for your location. If you ever have any doubts about Hebcal’s times, consult your local halachic authority.

Since candle lighting times are determined from latitude and longitude, a process called geocoding is used to determine the geographic position of a zip code.

Our zip code database comes from the 1999 ZIP Code file from the U.S. Census Bureau’s Gazetteer project.

The Gazetteer data from 1999 is much more complete than their data from 1990 — but still not as good as the commercial-quality data you get from the U.S. Post Office. Gazetteer contains zip code, city name, latitude and lognitude but is missing time zone.

Fortunately, the Gazetteer data contain a FIPS code (which represents the county a zipcode is part of). The U.S. National Weather Service provides county-timezone data (keyed on FIPS code), which contains timezone and Daylight Saving Time data for each county. By joining these two data sources, we can get a complete picture of each zip code.

Candle-lighting and Havdalah times are available for the following 54 cities:

Example: how to find candle-lighting times for San Jose, Costa Rica.

1. Go to the Getty Thesaurus of Geographic Names at http://www.getty.edu/research/tools/vocabulary/tgn/
2. Type “San Jose” in the Find box and click “Search” button
3. Click on the link that says “San Jose, Costa Rica, inhabited place”
4. Note the latitude/longitude represented in “degrees minutes direction” (in the example of San Jose, Costa Rica it is Lat: 09 56 N and Long: 084 05 W) and write this information down on a sheet of paper
5. Go to the Hebcal Interactive Jewish Calendar at http://www.hebcal.com/hebcal/
6. In the “Candle lighting times” section, click on the link that says “select by latitude/longitude“
7. Type the latitude and longitude into the form (9 degrees, 56 minutes North Latitude, 84 degrees 5 minutes West Longitude)
8. Select the Time zone and Daylight Saving Time options specific to your location
9. Click “Get Calendar” button at the bottom of the form