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 US and most of Europe 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...
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 Alps.
Locations close to the poles have two issues. And there's a third which specifically affects Antarctica.
The first issue is that at latitudes above the Arctic Circle and below the Antarctic Circle, there are periods of time when either the sun never sets or the sun never rises. The calendar copes with this reasonably well, although the dates when permanent night or day start and finish may not always be entirely accurate.
A second problem, and one which there isn't a solution for at the moment, is on the dates when permanent sunlight starts and finishes. In theory, once a year there is a day at the start of summer when the sun rises but doesn't set. And then, at the end of summer, there is a day when it sets, despite it not having risen.
However, if you find a date and location where the change to 24 hour sun begins, you'll see that the sun appears to set (very late) on one night, but then the following morning is presumed to have already been up at the start of the day. And, at the end of summer, you'll find a day of 24 hour sun followed by one with a sunrise, which is technically impossible!
The reason for that is that this site uses the built-in "date_sun_info()" function of PHP to calculate sunrise and sunset times. And that simply can't cope with cases where a date does not have either both a sunrise and a sunset, or neither. If one or the other is missing, it assumes that both are.
(It actually gets even more complicated than that in real life, because depending on the time zone and the actual longitude, it's possible to have a day where the sun rises very early in the morning, then sets very late at night, and then rises again almost straight away but before midnight - so you have a day with two sunrises. Which is another reason why the software simply gives up at that point.)
The concept of a normal 24 hour cycle of day and night is severely distorted in Antarctica, as almost all of the continent experiences periods of either perpetual night or perpetual daylight depending on the season. So, while there are a number of time zones covering Antarctica, these tend to be based on territorial claims rather than local conditions. And research bases often ignore these anyway and operate according to the time zone of their "home" country.
What this means in practice is that, during periods of the year where there is both day and night, it's possible to have the sunset before the sunrise - the day starts off in daylight at midnight, then the sun sets in the "morning" and rises again in the "evening". And the local time used by reearchers may have nothing to do with that anyway.
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.
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.
This is generally considered the limit of useable daylight; before dawn or after dusk artificial light is needed for most common outdoor activities. Many legal jurisdictions use the start and finish of 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.
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.
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