A good photograph is knowing where to stand - Ansel Adams

Solar Calculator 2.0 FAQ
Thursday, 14 July 2011 22:16

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Firstly, thank you for taking an interest in the shiny new Solar Calculator! My goal has always been to develop an easy to use solution for photographers who simply want to know where the sun (or moon) is going to rise and set on their next shoot. By necessity though, such an application was always going to be highly complex, and although I think the Solar Calulator is one of the easiest to use applications of it's type, there are undoubtedly many questions in the mind of a first time user.

With this in mind, I've prepared a Frequently Asked Questions list, which I hope will help ease you into the application, and help you get the most out of it.

  • Where do I start? The Solar Calculator is pretty easy to use... You can either search for a place name, or drag and drop the red marker to the location at which you're planning your shoot. This will automatically fill in the latitude and longitude for you. You can then enter the date your going to be shooting at that location, and click calculate. The Solar Calculator will give you a time of the sunrise and sunset on that day, as well as bearing (in degrees) to the location in which the Sun will rise and set.
    If you're not inclined to carry a compass with you on the day of your shoot, you can also just take a look at the map. The map window will give you an easy to read graphical representation of the location of the Sunrise and Sunset, as well as the current position of the sun at the time you've specified. You can also click the 'Satellite' button in the top left, to overlay the information on satellite photographs of the area you're shooting in.
    Also remember that anything the Solar Calculator can do for the Sun, it can do for the Moon also. To declutter the map, it does not show bearing markers for the Moon by default, however you can view this information by selecting Show from the 'Show/Hide Bearings on Map' dropdown box.
  • How does it work? The calculations used internally in the Solar Calculator are based on VSOP87, which is a planetary theory built by P. Bretagnon and G. Francou in the Bureau des Longitudes. With this theory, they produced data files, which have been used in Solar Calculator 2.0 to compute the coordinates of the Sun and Moon.
  • What new features are included? The new Solar Calculator has a whole host of new features. First and probably foremost, it is now able to calculate lunar (Moon) positions, as well as the Sun. Also included is automatic detection of magnetic declination for the current locations, with the ability to account for it in the reported bearings.
    From feedback recieved, one of the least favourite 'features' of the old Solar Calculator was the necessity of entering in the Time Zone (including whether or not DST was in effect) manually for each new location. This is now handled automatically for each location.
    Last but not least, accuracy has been improved massively. I had reports of the old Solar Calculator getting it wrongfrom time to time, especially in more Northern latitudes. It's new VSOP87 based algorithms have held up extremely well under testing, with a level of accuracy an order of magnitude about the previous version.
  • The Autodetect Time Zone feature gets it wrong at location X, Y and Z. Unfortunately, the time zone in effect at any particular latitude and longitude on the planet is not easy to determine automatically. The zone boundaries do not follow lines of longitude, and can be further complicated by things like daylight savings. This is well illustrated by this map at Wikipedia.
    The Autodetect Time Zone feature of the Solar Calculator is there as a convenience, and relies on data from AskGeo.com in order to determine the time zone at your requested location. I think they do an admirable job, but I have noticed that they get it wrong from time to time. In the unusual event they do get a timezone a little wrong, please be understanding and enter your time zone manually.
  • What is magnetic declination?. To sum it up, magnetic declination is the difference in the compass bearings between Magnetic North and True North at your location.
    Unless you've ever had to navigate by map and compass, you've probably never had cause to worry that a compass does not in fact point North, but rather to the Northern Pole of the planet's magnetic field. The Magnetic Pole moves from year to year, and is almost never located in the same place as the 'true' North Pole.
    The difference in the bearings between the two Poles is often only a few degrees, but if accuracy is important, then it's essential to allow for this when taking a measurment with a compass. The Solar Calculator has been designed to make this calculation easy for you, by plotting (and even predicting) what the magnetic declination is at the location you've selected, and giving you the option to allow for that in the bearings it gives.
  • Why are magnetic declination predictions only available for dates between January 1st 2000 and December 31st 2015?. The magnetic declination predictions the Solar Calculator makes are based on the US National Geophysical Data Centre's five-year geomagnetic field models. Currently, I only have the three field models from 2000 onwards available to me. If you have the coefficients files for earlier models I'd be happy to include them, so by all means get in touch.
    The NGDC explains why the models are only accurate within these dates here:
    Because the Earth's magnetic field is constantly changing, it is impossible to accurately predict what the field will be at any point in the very distant future. By constantly measuring the magnetic field, we can observe how the field is changing over a period of years. Using this information, it is possible to create a mathematical representation of the Earth's main magnetic field and how it is changing. Since the field changes the way it is changing, new observations must continually be made and models generated to accurately represent the magnetic field as it is. Source: http://www.ngdc.noaa.gov/geomag/models.shtml.
  • Why do the angles on the map not change when I enable magnetic declination?. Put simply they don't need to; magnetic declination is only a factor when taking a bearing with a traditional compass. Since the bearings plotted on the maps are based against True North, rather than Magnetic North they are not effected by the Earth's rather inconveniently lop-sided magentic field!
  • Why are tidal predictions only linked from US or UK locations?. Unfortunately, tidal prediction is a lot more difficult that plotting the positions of the Sun or Moon. The times and heights of the tides owe a lot to local geography as well as the predominant currents and weather conditions in the area. For this reason, it's simply beyond my resources to calculate myself, I've therefore had to link to third-party sites.
    Currently, NOAA Tides & Currents and UKHO Admiraly Easytide are the only sites that sites that have the locations of their tidal observation stations plotted and linked into the Solar Calculator.


Please feel free to email me at: questions@iesmith.net if you have any questions...