This page describes an algorithm that will enable you to calculate the time of Moon rise and set and Sun rise and set, as well as the twilight times, for any latitude on the Earth's surface. The algorithm presented here will warn you if the Moon or Sun remains above or below the horizon all day (a definite possibility above latitudes of 66.5 degrees North), or if the object does not rise or set within the day chosen (the Moon will exhibit this behaviour on one day in each lunar cycle).
If you just want the results for your observing location, then I recommend the Web page based programs provided by the US Naval Observatory. One of these programs will give you a compact yearly calendar style printout of rise and set times at a given location on a single sheet of paper. I use a set of printouts in my own observation planning - its easier than switching on the computer!
In the implementation of the algorithm on this page, I have used approximate routines to calculate the coordinates of the Sun and Moon. You can choose latitudes where the program will predict a very short day when the Sun really remains below the horizon all day - this inaccuracy affects latitudes close to 67.43 degrees North, and you can imagine a 'fuzzy zone' around the Earth at this latitude (and a mirror image one in the South) where the program may be in error. I suspect that more accurate position routines and better approximations for topocentric parallax, refraction and the diameter of the Sun and Moon would shrink this 'fuzzy zone', at the cost of increased complexity.
The algorithm is implemented in
QBASIC, a well
structured interpreted basic available free for Windows computers, and
easily adapted to Visual Basic syntax. The implementation is based very
PASCAL routines taken from Montenbruck and
The rising and setting of the Sun and Moon is defined here to be the instant when the upper limb of the object is seen to cross a mathematically flat horizon by an observer at sea level. Real observing situations on land rarely approach this idealised model, and for my uses calculating rise and set times to the nearest minute is fine.
The following factors need to be addressed
In Montenbruck and Pfleger's approximate implementation, the last three points above boil down to subtracting a single correction term from the geocentric altitude of the Moon or Sun when looking for rise or setting events.
The large difference between the Moon and the Sun correction reflects the much larger parallax effect on Moon positions - roughly 1 degree for the Moon compared to 9 arc seconds for the Sun.
Possible algorithms include:-
I used the 'single position and calculate as for star' method on a programmable calculator - the results for the Sun at moderate latitudes were 20 minutes or so wrong. Given the state of the local horizon, this was not much of a problem. This 'method' was very poor for the Moon, almost worse than guessing from the number of days since New Moon.
The 'trial and improvement' method with allowance for refraction, parallax and disc size is much better, and can give good results for the Sun for latitudes below 65N. The method will mostly converge after a few iterations - see the spreadsheet implementation on this Web site, or Paul Schlyter's page.
Tral and improvement methods get fiddly in situations where the object remains above or below the horizon for a number of days, or where the object does not set one day or the next.
A table based method could be very simple - just calculate the altitude of the Moon for each minute of the day and find when the altitude changes sign. There is your rising or setting event.
This simple table based method could detect the 'always above the horizon' or 'always below the horizon' situations as the (corrected) altitude would never change sign over the day. The method could also cope with the Moon missing a rise or set each lunar cycle, but 'grazing' situations, where the Sun never quite dips below the horizon, would be more difficult as there would be no sign change in the altitudes. A real program based on this system would be really slow, requiring at worst case 1440 calculations of altitude for each object for each day.
Interpolation based methods use a shorter table of altitudes - in my implementation we use a table with a maximum of 12 rows - and fit a quadratic function to each set of three altitudes in turn. We then use some GCSE algebra to find the times when the quadratic crosses the X axis - has zero altitude - and these times are candidates for a rising or setting event. This system can also deal with the situation when the rising and setting occur within the hour - the quadratic then has two zeros. Finally, if the two zeros are the same, then there must be a 'grazing' event in that interval, and the algorithm can detect the event.
The steps in the procedure are
You can download the tested
QBASIC code below. The
QBASIC editor insists on adding functions and subroutines
to the end of the listing in alphabetical order - which does not make
reading the code any easier. I have added comments in the 'main' program
that should help you link the code to the description of the algorithm
Below are some samples of program input and output, together with accurate results from Chris Marriott's SkyMap Pro 6.0 and with a year 2000 calendar for the Sun, Moon and Nautical Twilight generated from the USNO web based program.
Rise and set times for today in Birmingham UK - simple case with no
ambiguities. Below is the dialogue with the QBASIC program. Times of
events are given in form
hhmm, and are UT times.
Rise and set for Sun and Moon ============================= Year (yyyy) - - - - - - - - :2000 Month (mm) - - - - - - - - :1 Day (dd) - - - - - - - - :3 Time zone (East +) - - - - :0 Longitude (w neg, decimals) :-1.91667 Latitude (n pos, decimals) :52.5 Moon 501 1409 Sun 818 1606 Nautical twilight 653 1731
Skymap Pro gives the following times for the Sun and Moon on 2000 Jan 3 - agreement is fine to nearest minute, except for Moonrise that rounds the wrong way to give a result one minute different from the USNO tables.
Sunrise and twilight Date Rise Set Naut Twi E Naut Twi B 03 Jan 2000 08:18:11 16:05:50 17:30:56 06:53:06 Moonrise Rise: 5h 0m 23s Set: 14h 9m 18s
QBASIC program gives a 'day' lasting 8
Rise and set for Sun and Moon ============================= Year (yyyy) - - - - - - - - :1999 Month (mm) - - - - - - - - :12 Day (dd) - - - - - - - - :25 Time zone (East +) - - - - :0 Longitude (w neg, decimals) :0 Latitude (n pos, decimals) :67.43 Moon 1747 1206 Sun 1156 1204 Nautical twilight 747 1613
Skymap Pro shows that the Sun remains below the horizon for the whole
of that day at the latitude given. SkyMap Pro gives the altitude of the
(centre of) Sun at transit for this day as roughly 53 minutes below the
horizon. This implies an error of roughly 30 minutes in Sun position at
that time for the
QBASIC program. This seems rather large
and I will be puggling about with this a bit more.
The Moon times and times for twilight are within a minute of those
produced by SkyMap Pro. You might treat output from the
QBASIC program presented here with caution for locations
with latitudes between 67 and 68 North.
For Narvik in Norway (further North), the program gives correct results give or take a minute:-
Rise and set for Sun and Moon ============================= Year (yyyy) - - - - - - - - :2000 Month (mm) - - - - - - - - :1 Day (dd) - - - - - - - - :3 Time zone (East +) - - - - :1 Longitude (w neg, decimals) :17.42 Latitude (n pos, decimals) :68.43 Moon 629 1157 Sun always below horizon Nautical twilight 743 1607SkyMap Pro gives
Moon Rise: 6h 27m 59s Set: 11h 57m 47s Date Rise Set Naut Twi E Naut Twi B 03 Jan 2000 --:--:-- --:--:-- 16:07:06 07:42:21
Oliver Montenbruck and Thomas Pfleger,
Astronomy on the Personal Computer
1994, 3rd edition
Section 3.8 of this book is the basis of the
listing here. I have translated the
PASCAL routines into
QBASIC one by one. This book does not contain an exhaustive
explanation of the quadratic fit method, and the test data the authors
provide will not test all branches of the code. This book comes with a
software disc with all the
PASCAL routines - but does not
cover all the ground in, say, Meeus' Astronomical
Sky and Telescope's software pages contain listings in 'spaghetti
basic' that implement a similar method for the Sun and the Moon - there
is a clever fix used to avoid re-calculating positions for the Sun and
Moon up to 12 times each day at the cost of some increase in the 'fuzzy
zone' when the programs may predict a Sunrise and set when there is
none. The links below are to the
Seidelmann, P. Kenneth (ed)
Explanatory Supplement to the Astronomical Almanac
University Science Books
1992, completely revised
Definitive reference on all aspects of the ephemeris and
associated calculations. No hostages taken, no example calculations and
modern vectoral notation used throughout. Sections 9.311 (p484) and 9.33
(p486) present the iterative algorithm for Sunrise for moderate
Duffett-Smith, Peter Good treatment of the basics of coordinate systems. Presents the
formulas for rise and set time of a star, and presents an interesting
linear interpolation scheme for Moonrise.
[ Root ]
Practical Astronomy with your calculator
Cambridge University Press
3rd edition 1988
Last Modified 2000 Jan 3rd
Good treatment of the basics of coordinate systems. Presents the formulas for rise and set time of a star, and presents an interesting linear interpolation scheme for Moonrise.
[ Root ]