Sunrise & Sunset Reckoner

Tutorial

• Theory
  an astronomical explanation of the sun's movement
• The Time of Sunrise & Sunset
  an explanation of Apparent Time and Clock Time
• How to use the Reckoner
  step-by-step instructions
• Disclaimer
  why you should never believe what you read in web pages
• A word about Declination
  where to go for compass accuracy
• The Reckoner

Theory

As the earth rotates, the sun appears to sweep across the sky from east to west, and the path it follows changes from season to season. In the northern hemisphere the sun rises at its most southerly point in midwinter - this is known as the winter solstice (December 21st) - and in mid-summer at its most northerly - the summer solstice (June 21st). Midway between those dates the sun rises due east and sets due west on the spring equinox (March 21st) and the autumn equinox (September 23rd). On those dates the sun rises at 6 a.m. and sets at 6 p.m. all over the world - the word "equinox" is from the Latin meaning "equal night".

The point of the heavens directly above the observer is known as the zenith, and a line drawn due south from this point to the horizon is known as the observer's meridian. The sun crosses the meridian at noon - at this point it is at its highest in the sky. The time before noon is known as ante meridian (a.m.) and after noon is known as post meridian (p.m.).

The angle on the compass at which the sun is seen to rise is known as the azimuth of the rising sun. Due north on the compass is represented by 0°, east is 90° and west is 270°. On either equinox the sun will rise at 90° and set at 270°; on the equator on that day the noon-day sun will be seen to be directly overhead (on the zenith).

On a globe the earth will be shown as divided into lines of latitude and longitude: the latitude is measured in degrees north (+) or south (-) of the equator (0°), the longitude is measured in degrees east (+) or west (-) of the Greenwich meridian (0°) which passes through London. The North Pole is at latitude 90° north, while the International Date Line is at longitude 180° west.

The Tropic of Cancer is a line at latitude 23½°N, and represents the northernmost point at which the sun may be seen on the zenith at noon (on the summer solstice). Similarily the Tropic of Capricorn (23½°S) is the southernmost point at which the winter solstice noon sun will be directly overhead.

On the Arctic Circle (66°N) at the winter solstice, the sun does not rise above the horizon, while at the summer solstice it is seen not to set. The opposite is true for the Antarctic Circle (66°S). North or south of these latitudes there will be longer periods of the year with perpetual night or perpetual day.

The Time of Sunrise & Sunset

The way to resolve this problem is to deal with two "types" of time:
Apparent Time and Clock Time.

On the equinox, the sun rises at 6 a.m. Apparent Time; the sun on any day is at its highest in the sky at noon Apparent Time. Clock Time can be very different: for example during the summer Britain (in common with many other countries around the world) shifts its clocks forward by one hour - so-called Daylight Saving Hours - so the sun will be at its highest at 1pm Clock Time. As another example, Iceland by choice sets its clocks to GMT, but Reykjavik is a full 22° west of Greenwich - so the sun rises on the equinox at 6:00am Apparent Time, 7:28am Clock Time.

Countries nearer the equator have no requirement for Daylight Saving Hours: their hours of daylight hardly change through the year. Countries that do change their clocks in summer do so on different dates: be warned!
Check for your Daylight Saving Hours here.

Notes

Symmetry applies to all the times and azimuths of sunrise and sunset: when the sun rises at 30° east of due north, it will set at 30° west of north, i.e. 330°.

When the sun rises at 8:00am (4 hours before noon), it will set at 4:00pm (4 hours after noon), Apparent Time.

How to use the Reckoner

• From the Reckoner Index Page, first find the area of the world, then the country, then click on the city of your choice.
• This will take you to the correct part of the sunrise/sunset pages.
  Take a note of the time correction for the city of your choice.
  The azimuths are moderately accurate; but read the notes below.
  The times in the charts are all Apparent Time, i.e. they are accurate for somebody standing on the meridian of that time zone.
  If you haven't already read the explanation of the difference between Apparent Time and Clock Time read it now.
• There are 52 sets of sunrise/sunset figures: one for each week of the year. The dates are European style: day first, then month. They apply to any year.
• The times of sunrise and sunset shown are Apparent Time. Convert from Apparent Time to Clock Time by adding (or subtracting) the time correction previously noted.
• Very important: check for Daylight Saving Hours.
  If the country name (or, in the U.S., the state name) is followed by an asterisk *, that country/state applies Daylight Saving Hours in summer. Double-check the dates when such hours start and finish (or click here). As an example, in the U.K. British Summer Time begins on the first Sunday after the Spring Equinox (March 21st) and ends on the third Sunday after the autumn equinox (September 23rd).
  During Daylight Saving Hours, add one hour to the times shown.

How it works

The Reckoner consists of just four pages of sunrise/sunset predictions, running in 10° intervals from 70° North to 50° South (contact us if you need figures for further north/south).

The azimuths on these pages are accurate for any longitude.
The times are all Apparent Time ; i.e. they would be accurate if you were standing on the "meridian" of a particular time zone.
The time correction figures given for each location are calculated from the degrees of longitude east or west of that time zone's meridian.

Declination

Consideration should be given to Magnetic Declination, which is the difference between True North and Magnetic North (Compass North). The figures in these pages are True North. In many parts of4 the world, there is little practical difference, but the closer you get to Magnetic North (or Magnetic South), the differences can make a large difference to the reading given by a magnetic compass. For example, the Magnetic Declination for Cape Town, South Africa, is a full 25° West: this means that the magnetic compass reading for North is 25° out.

For an explanation, see: Geological Survey of Canada: Magnetic Declination and to find the declination for your area, see: GSC Canadian Geomagnetic Reference Field (you will need to know your latitude and longitude). If the declination is 10° West, add 10° to the compass reading; if it is 10° East, subtract 10° from the reading.

Disclaimer

The primary purpose of these pages is as a reckoner, and it is not designed to be used in situations where the azimuth or time of sunrise or sunset is critical. Nonetheless for most practical purposes the results can be relied upon, but bearing in mind the following points:

1) The azimuths are accurate for an observer standing at sea level and looking at a nautical horizon. It is important to note that allowance must be made for elevation above sea level and local horizon (e.g. mountains, buildings etc). The angle of the sun's movement is oblique to the horizon in northern and southern latitudes, particularly in summer; the apparent azimuth of sunrise/sunset will thus vary according to local conditions. This effect is least important near to the equator, where the sun effectively rises and sets at right angles to the horizon.

2) The compass angles given are True North: no allowance is made for local variations in magnetic north (see Magnetic Declination above), nor for refraction, which varies according to climate and local weather conditions.

For true accuracy, there is no substitute for personal observation. Whilst it is true that Astronomical Institutes will supply accurate azimuths in degrees, minutes and seconds, local conditions as described above will affect the usefulness of the result. It is better to use these figures as a rough guide and then arrange to be there on the spot ready to respond to local conditions.

© Location Works 2004