Determining the Hebrew Day

A Hebrew Day begins at sunset. The time of sunset is defined scientifically as the moment the trailing edge of the sun's disc disappears below the horizon to the west as seen by an observer at a particular location on Earth. At sunset, the true astronomical position of the sun is about one sun diameter below the horizon, yet delayed visibility occurs due to the bending of light through the atmosphere as a result of refraction. Sunset should not be confused with dusk, which is the moment at which darkness falls when the sun is about 18 degrees below the horizon.

In some places on Earth the sun never sets below the horizon during certain times of the year. Midnight sun occurs in latitudes north and nearby to the south of the Arctic Circle, and south and nearby to the north of the Antarctic Circle where the sun remains visible at local midnight. If the weather is fair, the sun will be visible 24 hours a day in those places. At the northern most point in Finland, the sun does not set for 73 days during summer. In the case of midnight sun, a Hebrew Day begins when the sun reaches the lowest point on the horizon at sundown.

The period of time between sunset and darkness is known as twilight. Science has established three subcatagories of twilight. Civil twilight is when the center of the sun is 6 degrees below the horizon. Nautical twilight is when the center of the sun is between 6 and 12 degrees below the horizon. Astronomical twilight is when the center of the sun is between 12 and 18 degrees below the horizon. In Hebraic thought, twilight has been defined as the time when two stars of medium magnitude are still visible. Upon the appearance of the third star, it is night. Twilight turns to night in an instanteneous moment. In Hebraic thought, this moment has been called "the twinkling of an eye."

Finding Your Local Sunset Time

Select YES in the Summer, NO in Winter.

How to Use the Sunset Calculator:

1. Select a location from the "Select a City" pulldown menu, OR select "Enter Lat/Long ->" from the pulldown menu, and manually enter the latitude, longitude and time zone information in the appropriate text boxes. The following sign conventions are used:
   
   

   Longitude:	Latitude:
   Positive in Western Hemisphere	Positive in Northern Hemisphere
   Negative in Eastern Hemisphere	Negative in Southern Hemisphere

   
   Latitude and Longitude can be specified using the "Degrees", "Minutes" and "Seconds" fields, or as decimal degrees entered only in the "Degree" field, leaving "Minutes" and "Seconds" fields blank. Selecting a city from the pulldown menu, automatically updates the "Latitude", "Longitude" and "Time Zone" fields. To input the latitude, longitude or time zone manually, be sure to select "Enter Lat/Long -->" from the City pulldown box, or your numbers will be overwritten by the selected city's location.



2. You can enter a different time zone for a location by selecting "Enter Lat/Long -->" in the City pulldown box. Otherwise, the time zone associated with the selected city's Local Standard Time will be automatically entered. Selecting "Yes" in the "Daylight Savings Time" field will cause the resulting sunrise, sunset and solar noon times to be adjusted forward by one hour.



3. The program retrieves the current date from your computer, and fills the month, day and year fields. To perform calculations for a different date, simply select a month in the pull down menu, and enter the day and four digit year in the appropriate input boxes. When entering a day or year, you will need to click the "Calculate Sunrise/Sunset" button to update the results for this date. Only dates within calendar years ranging from 1900 C.E. to 2014 C.E. are allowed.



4. For locations above the Arctic Circle and below the Antarctic Circle, when a sunrise or sunset does not occur on the given day the program locates the local time and date of the most recent sunrise or sunset, and the next sunset or sunrise. When this occurs, the Universal Times fields will display "prior" or "next" information instead of actual Universal Times (UT times).



5. NOTE: For latitudes greater than 72 degrees N and S, calculations are accurate to within 10 minutes. For latitudes less than +/- 72° accuracy is approximately one minute.