How accurate are candle lighting times?

Candle-lighting and Havdalah times are derived from sunset times, which are approximated from a location (latitude, longitude) and day of year. As of August 2020, Hebcal.com uses a sunset algorithm published by the US National Oceanic and Atmospheric Administration. The NOAA claims accuracy within 2 minutes except at extreme northern or southern latitudes.

Differences of 1-2 minutes between Hebcal and other sources publishing candle lighting times or sunset times are expected. Remember that candle lighting times can only be approximated based on location. Here are a few common reasons why you may see differences in candle lighting times:

  1. Different minhag on when to light candles. Hebcal defaults to 18 minutes before sundown for most locations (40 minutes before sundown for Jerusalem). Other sources may use 20 minutes before sundown. Hebcal gives an option to specify a different number of minutes before sunset if you don’t follow the 18-minute minhag.
  2. Slightly different sunset calculators. The sunset calculator we use on Hebcal.com as of August 2020 uses an algorithm published by the US National Oceanic and Atmospheric Administration. Hebcal’s NOAA algorithm is implemented in JavaScript using double-precision floating point arithmetic. Other sources may use a slightly different algorithm, for example one by Jean Meeus. However, even if the other source also uses the NOAA algorithm, the implementation could differ slightly; there are many constants and opportunities to round and truncate which could result in slight differences in the final calculated sunset time for a given day and location.
  3. Slightly different latitude/longitude definitions for a given city. Since 2013, Hebcal.com has been using lat/long definitions from GeoNames.org, which is available under a Creative Commons license. For the USA, we purchase a commercial ZIP code database from zip-codes.com. For very large cities, the sunset at the east side of the city might be a minute earlier than the west side of the city.
  4. Deliberate rounding down (or up). Hebcal rounds candle lighting (Friday) times down to the nearest minute, and rounds Havdalah times up to the nearest minute. To be more precise, we use the floor minute for candle-lighting, and we use the standard mathematical rounding rule for Havdalah. The idea here is that it’s better to display candle-lighting time as much as 59 seconds earlier than strictly necessary, and for Havdalah it’s better to wait an additional 30 seconds to end Shabbat/yontiff.
    • For example, if the exact candle lighting time from the sunset engine (including seconds) was at 20:02:31 or even 20:02:59, Hebcal displays candle-lighting as 20:02.
    • On the other hand, if the Havdalah calculation is 21:17:29, Hebcal will display 21:17. If it were 21:17:30 through 21:17:59, Hebcal displays 21:18.

As of August 2020, options for Havdalah times have also changed. Hebcal now offers an option to use tzeit hakochavim, the point when 3 small stars are observable in the night sky with the naked eye (sun 8.5° below the horizon). This option is an excellent default for most places on the planet. We also offer the option to use a fixed number of minutes past sundown. Typically one would enter 42 min for three medium-sized stars, 50 min for three small stars, 72 min for Rabbeinu Tam, or 0 to suppress Havdalah times.

Lastly, remember that the NOAA 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.