Some of the more commonly asked questions
- How accurate are the times shown on this site?
Reasonably accurate. This site uses the built-in PHP functions for calculating sun positions, and these are based on the standard published algorithms. The algorithms get progressively less accurate the futher away from the equator you get, but for the UK the results ought to be correct to within a minute or so. However, this is for ideal conditions, which, in reality, are rarely ideal. See the next question for more information...
- Why do the times shown here differ from my own observation?
The calculated times are accurate, to around a minute, for ideal conditions. However, they take no account of local geography. In particular, the calculations assume a perfectly flat area at precisely sea level, and the higher or more mountainous your locality the more inaccurate the times will be. So it's fine for people who live in the Fens, but less useful for those who reside in the Cairngorms.
- I thought the equinoxes were on 21st March and 22nd September, yet the calendar
shows equal lengths of day and night on different days. What's going wrong?
The equinoxes are indeed on 21st March and 22nd September, but that's not when the lengths of day and night are equal. Confused? Read on...
The equinox is defined as the time when the sun crosses the plane of the earth's equator, so that both hemispheres get an equal amount of light. If the sun were a perfect point source, and the earth had no atmosphere, this would also be the time when the day and night are equal in length. However, the sun isn't a point, it's a noticable sphere, and the earth does have an atmosphere. The atmosphere bends the sunlight a bit, allowing us to see it a bit before we would without an atmosphere (in the morning), and a bit after (in the evening) than we would if the earth were airless. Also, sunrise is defined as the time at which the leading edge of the sun crosses the horizon, and this happens a little before the centre of the sun is visble. The opposite occurs in the evening – sunset occurs when the trailing edge of the sun crosses the horizon, not when the centre does so.
Apart from meaning that the day and night are not equal in length on the equinoxes, as you may have (incorrectly) learned at school, what this also means is that overall, the earth gets slightly more daytime than night. The longest day (in summer) is longer than the longest night (in winter), which is good news for SAD sufferers but possibly not so good news for bats.
The correct term for the date on which the lengths of day and night are equal is equilux, from the latin for "equal light". Unlike the equinox, the date of the equilux varies with location and geography, so it won't be the same for everyone at the same time. At the equator and close to it, there is never an equilux as the length of the day is always greater than the length of the night.
- What's the difference between "dawn", "dusk" and "twilight"?
Strictly speaking, twilight is the period of time either before sunrise or after sunset when there is a noticable amount of sunlight in the sky, while dawn is the start of that time in the morning and dusk is the end of that time in the evening. In other words, morning twilight starts at dawn and ends with sunrise, then in the evening twilight starts with sunset and ends with dusk. But the words are often used interchangably, and no-one other than a pedant is going to care if you get them the wrong way round.
- What's the difference between civil, nautical and astronomical twilights?
- Civil Twilight starts (in the morning) or ends (in the evening) when the sun is 6 degrees below the horizon. This is generally considered the limit of useable daylight; before civil dawn or after civil dusk artificial light is needed for most common outdoor activities. Many legal jurisdictions use the start and finish if civil twilight as a marker for the "official" transition between day and night (for example, in the concept of "lighting up time"), although in such cases it's usually defined in terms of a time before sunrise or after sunset rather than in the sun's position in degrees. By the time of civil dusk, most street lights will have come on and it will start to "feel" dark – there will still be a lot of light in the sky, but the perception is that it's the beginning of the night rather than the end of the day.
- Nautical Twilight (also known as Military Twilight) starts or ends when the sun is 12 degrees below the horizon. By the end of nautical twilight, the horizon will be indistinct and unlit ships will no longer be silhoutted against the sky. In practical terms, the limits of nautical twilight also mark the limits of unassisted human vision. The start of nautical twilight in the morning is also referred to as "first light", and the end of nautical twilight in the evening is known as "nightfall".
- Astronomical Twilight starts or ends when the sun is 18 degrees below the horizon. This marks the point at which there is no longer any detectable sunlight refracted in the atmosphere, and hence when the time is most suitable for astronomical observations. To the naked eye it will appear completely dark well before (or after) the end or start of astronomical twilight, but telescopic observations will reveal the presence of refracted light. During the summer months, the sun doesn't get far enough below the horizon in the UK for Astronomical twighlight to end at all.
- Why can't the map locator page automatically get the right timezone
(or the country name) for where I click on the map?
Because that information isn't part of the data returned by the Google Maps API. Clicking on the map returns a latitude/longitude pair, and nothing else – all the rest has to be calculated. Converting a latitude/longitude pair into a named location (such as a country name) would mean having a database of coordinates for all the national borders in the world, and as far as I can tell such a thing doesn't exist (at least, not in a freely available form).
- The drop-down box for selecting a timezone doesn't default to my own location. Why not?
Assuming that your PC has the date and time set correctly, then the script will detect your offset from UTC and attempt to pick the most likely timezone. However, there are often multiple timezones with the same offset, the difference between them being the way they handle daylight saving time, so if there are multiple zones to choose from then the script has to make something of a guess. And sometimes it gets it wrong.
- Why does the script sometimes get the wrong start and finish dates for daylight savings time?
Because it doesn't know any better. Literally. There is no simple formula to generate the start and finish of daylight savings time, as individual nations can (and do) change them for whatever reasons they feel like. Even where the dates are generally fixed, they can be adjusted for special occasions. For example, Australia changed the dates of daylight savings time to cover the Sydney Olympics. There's no way to get round this other than by maintaining a list of dates and updating it regularly. The list on the server is as up to date as the most recent installation of the PHP PECL timezonedb extension.
- Why doesn't the UK Postcode page recognise my postcode?
Because it's not in the database. There are around 2.5 million postcodes in the UK, of which we've got about 1.9 million in the database here. Most of the missing ones are non-geographic postcodes which don't have a latitude and longitude anyway, but that still leaves several thousand which ought to be in the database but aren't. Unfortunately, there is no free, legal and simple way to update these automatically. The official postcode data is available from Royal Mail, but they charge an arm and a leg for it and, since this website generates no income other than the occasional click of a Google ad, paying anything more than a little finger would be unrealistic. In any case, the Post Office data doesn't include latitude and longitude (it uses OS grid references for geographic mapping, which is fine for things like route and distance finding but not so helpful for this type of application), so I'd still need to do a lot of additional processing to make it useful. So, at the moment, the postcode database is essentially fixed and will gradually get more and more out of date as time goes by.
One possible way forward is to use the Google Maps API to geocode postcodes instead of relying on an internal database. The advantage of that is that it should be possible to generate the correct sunrise/sunset times for pretty much anywhere in the world based on a standard postal address. Depending on the circular tuit supply, that may or may not be the next project for this section of the website.