Updated:  21 May 2015

# Help with Astronomic Computations

## How to Compute Moonrise & Moonset Times

### Using the data entry form:

1. Enter the latitude and longitude of the location as degrees & minutes. An error of about 15 minutes in latitude or longitude may result in an error of several minutes in the computed time but this varies with location and date.
2. Select the time zone for the required location. Selecting an inappropriate time zone may result in unusual times.
3. Enter the date for the event in the order - day, month, year, each separated by a /. If you enter just the year (4 digits) the event will be computed for every day of the year. If you leave this entry blank the current date will be used.

## How to Compute Sunrise, Sunset & twilight Times

### Using the data entry form:

#### Step by step instructions:

1. Enter the name of the location (This is for display, printout purposes only.
2. Enter the latitude and longitude of the location as degrees & minutes. An error of about 15 minutes in longitude will result in an error of about 1 minute in the computed time. An error of about 15 minutes in latitude will generally result in an error of less than 1 minute in the computed time but this varies with location and date.
3. Enter or select the time zone for your location. Selecting an inappropriate time zone may result in unusual times.
4. Select the event required.
5. If a user defined event was selected (user defined horizon), enter the degrees and minutes of the zenith distance (the vertical angle from directly overhead to the desired horizon), eg the ideal horizon has a zenith distance of 90 degrees while a zenith distance of 95 degrees gives a position 5 degrees below the ideal horizon.
6. Enter the date for the event in the order - day, month, year, each separated by a /. If you enter just the year (4 digits) the event will be computed for every day of the year. If you leave this field blank the current date will be used.
7. Select COMPUTE to obtain the answer, or RESET to clear all the fields.

## How to Compute the Position of the Sun or Moon

### Using the data entry form:

1. Enter the latitude and longitude of the location as degrees & minutes.
2. Select the time zone for the location. Selecting an inappropriate time zone may result in unusual answers.
3. Select either Sun or Moon as the object whose position you wish to calculate.
4. Enter the date and time for the event exactly as: dd/mm/yy,hh:mm:ss The date and time given must be in terms of the Time Zone previously entered. If you do not enter a date or time, the current date and time will be used.

The accuracy of the computed azimuth and elevation is better than 1 minute of arc.