Why do we need to add 5 minutes to sunset times?
(Mufti) Amjad Mohammed
(Maulana) Yousaf Baig
(Maulana) Farid Patel
I have seen some Ulama recommending to add 2–4 minutes to sunset times and other Ulama strongly condemning this recommendation? Why should 2–4 minutes need to be added to absolute times obtained from Royal Greenwich Observatory?
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh
We would like to begin by extending our gratitude to you for asking this question. May Allah (SWT) give you the best of rewards (Ameen).
We are assuming that you are using accurate figures from a reputable site such as HMNAO. We recommend that you add 5 minutes to the listed sunset times, for the reasons explained below.
[2:187] And eat and drink until the white thread of dawn becomes distinct to you from the black thread [of night]. Then complete the fast until the sunset
Refraction of Light:
When light passes from one medium (material) to another, it changes speed. This is because the speed of a light wave is determined by the medium through which it is passing. This change in speed causes the light to bend (refract) and it is a well known and observed phenomenon. Observable refraction is demonstrated in the following picture.
Light from the sun is similarly impacted (bent) on its way to earth by several factors. Pollution within our atmosphere affects the refraction of light 1 . and is different at various places on Earth. In addition to pollution, the extent of the refraction also depends on atmospheric turbulence, including air temperature and atmospheric pressure: the higher the pressure and the lower the temperature, the larger the refraction angle. So, if you watch the sun set in an area of high pressure on a cold day, you may have to wait several seconds for the upper edge of the sun to disappear behind the horizon, compared to a day with average pressure and temperature 2. Although pollution may have an impact on refraction, turbulence in the Earth’s atmosphere has a much higher impact on refraction of light, particularly temperature variations in the atmosphere. 3
When it comes to sunset for the establishment of Maghrib time, you have to account for refraction because the physical sun is actually lower than what appears to our eyes. 4
Why was 4 minutes recommended before?
The phenomenon of sunrise/sunset times not being accurate was first raised in the 1990s. In a research paper by Dr Bradley E. Schaefer (1990) 5 he explicitly states that “the time of sunrise can only be predicted with an accuracy of 4 [minutes]”, and sunset times may also suffer from inaccuracies. Science is driven by observations and research and in 2018, we have more precise observational data for high latitude countries.
We have noticed that in some of the programs standard refraction is already accounted for e.g. HMNAO data sheet (7) states:
The sunrise and sunset times that are prepared by Her Majesty’s Nautical Almanac Office correspond to the instants times when the zenith distance of the centre of the Sun is 90° 50 ́: this allows 34 ́ for refraction in the atmosphere and 16 ́ for the angular semi-diameter of the Sun. (The zenith distance of an object is the angle between the direction to the zenith (i.e. the point vertically overhead) and the direction to that object.) 6
Our Sun Calculator, which shows sunrise and sunset times in a location of your choice, takes the effect of refraction into account. Our calculations are based on the standard atmospheric pressure of 101.325 kilopascals and temperature of 15°C or 59°F, so sunrise and sunset may happen some seconds before or after the stated time when the temperature or pressure strongly deviate from the average.
Here are the sunrise/sunset times for a location in Bradford which is at 53°47’38.08“N, 1°45’7.42“W:
|Date and Time||05:40||20:30|
Variance in (calculated) sunset timings across geographies & seasons
While there are scientific studies on this topic from various regions around the world, there is no consensus on how refraction affects sunset times in (specific parts of) Britain. 7. Having said that, raw data from Alberta, Canada has demonstrated a seasonal change in the observation of sunset with greater levels of refraction around winter compared to summer 8 When this refraction data is used in the U.K. setting we find the value varying from 4 minutes to 7 minutes. We appreciate this may not be wholly accurate due to the difference in setting, but until a similar study is conducted in U.K. we will utilise it; especially as it suggests a later time which is more precautionary when it comes to acts of worship. For instance, we can add that similar research carried out in Holetown, Barbados 9 found the refraction to be lower suggesting a further two minutes deducted from HMNAO would be sufficient. Returning to the Canadian data, once the two minutes which HMNAO deduct to take Astronomical refraction into consideration we are left with an additional 2 to 5 minutes. We have consulted two experts in this field and their opinions have differed on the matter. Therefore to take seasonal effects into consideration and to be as precautionary as possible without increasing the time unnecessarily, also to issue a standard value for the year we recommend adding five minutes to the HMNAO time for sunset. We have confidence from those astronomers that we have consulted that we will be well within the correct timeframe. This is an ongoing research pursuit for Wifaqul Ulama and more research will be presented in due course.
Therefore, we suggest that the addition of five minutes to the HMNAO time to be more precautionary and hence recommend that based on current understanding.
جزاك اللهُ خيرًا
Dr Victor P. Debattista
Hi Dr. Debattista,
I spoke to you a couple of weeks ago on the phone. We do have a technical query and we were wondering if you can answer it for us.
We have looked at HMNAO sunset times and understand that they account for refraction in their calculations. However, we believe the calculations to be standard and static.
The Islamic tradition is to actually observe the sunset but normally Muslims take their times from the Observatory.
In classical application, a person would be observing the sunset and then opening their fast when the sun is no longer visible. Muslim need to use the sunset times to open their fasts during Ramadhan (after sunset) and to pray etc. In your opinion, is our recommendation of adding two minutes sufficient to the HMNAO times to account for the impact of refraction? This query is specifically for the times for Britain.
Our recommendation is purely as a precautionary measure to ensure that the sunset has occurred.
Does air pollution have any impact on HMNAO sunset times?
Can we publish your answer on our website with your answer?
We have asked this of a number of experts and will be looking at all of the replies comprehensively.
Thank you for your question.
The calculation that is being done by HMNAO, as per your link, requires that the Sun is half a degree below the horizon, which they determine means that the Sun is no longer visible even with refraction taken into account. The thing to keep in mind about refraction is that it is highly dependent on atmospheric conditions. Not so much pollution as turbulence (wind etc) and, especially, temperature variations in the atmosphere. These are basically weather patterns and, as usual, vary significantly by day and by location and are essentially impossible to predict with certainty. A search online showed that refraction as large as 2 degrees has been recorded historically!
It is worth pointing out that this was in Antarctica where one might expect temperature variations in the atmosphere to be more extreme.
So my conclusion as an astronomer is that allowing a further two minutes for the Sun to set is a good idea as a precaution but that one should always be aware that extreme (therefore unlikely but not impossible) conditions may result in the Sun reappearing when it should be well below the horizon. Another issue to be aware of is that sunset varies with altitude. At 12 km above the surface, as on a plane cruising, the horizon is 2 degrees lower, which means the Sun sets about 1.5 minutes later (in the UK). This probably affects a small number of people but it might be worth drawing your attention to it.
I hope this helps with your decision making. Please do let me know if you need more help with this and I will try my best to help you.
Yes, the correlation between sunset and latitude and the extreme scenario of the sun reappearing again is noted.
However we are making an assumption that the person is at ground level in the British isles and observing sunset on a typical day with no abnormal deviation.
Based on our assumptions, we would like to recommend adding extra two minutes to HMNAO times to British Muslims and I hope that in your opinion this will be sufficient?
And can we add your reponse with your name to our site?
Indeed, adding a further two minutes would ensure that any fluctuation due to turbulence, pollution or other factors is compensated for. You can add my name to the site.
(Dr) Victor P. Debattista
Professor of Astrophysics
Thank you for getting in touch — that is a very good question! Unfortunately, there is no simple answer.
First of all, it depends a little bit on exactly what you mean by “sunset”. The obvious definition is simply “when the sun has disappeared below the horizon”, but that will clearly depend on your surroundings — sunset will appear to be much early if you are in a valley surrounded by mountains, than if you are looking out to sea. In astronomy, we tend to choose a more consistent definition which is, in effect, “when the sun drops below the horizon that you would have if you could see the sea”. That still depends on your altitude (it will be a bit later if you are higher up) but it is more consistent and can be calculated accurately for any place on the surface of the Earth if the altitude is known.
However, there are some things that you cannot calculate in advance. The main one is refraction, as you say. When the sun seems to disappear below the horizon is actually later than when it really did drop low enough in the sky, as the light is bent by refraction through the air around the surface of the Earth and so you can still see the Sun a bit after it should have been blocked by the horizon (forgive me if I am telling you things you already know — I just want to make sure we are both talking about the same thing).
Unfortunately, the amount of refraction depends on a number of factors, some of which cannot be predicted. For example, the air pressure is important since that changes the “amount” of air between you and the horizon and hence the amount of refraction. Therefore, when calculating sunset time, we have to make an assumption. The simplest thing to do is assume there is no air (and hence no refraction) but that is very inaccurate, so we normally just assume a “standard” air pressure. That gives an answer which is correct on average, but will be a bit inaccurate on any given day.
To make things more complicated, as you move away from the equator, the angle that the Sun sets moves further away from the vertical (compared to the horizon) and changes during the year, so the amount of difference in sunset-time that a given amount of refraction makes varies from place to place and during the year.
For astronomers, we tend to deal with this by being pessimistic about sunset times. Since we are only really interested in knowing when the sky will definitely be dark, we tend to take the worst case, which around the UK usually means a maximum of about 6 or 5 minutes being added to sunset time from the “average refraction” calculations. However, such long delays are very rare — you are more likely to see no more than a couple of minutes extra delay, so your suggestion of adding 2 minutes would usually be fine.
Since this is not entirely predictable (even with quite a good weather forecast) you are probably going to have to decide what uncertainty you are comfortable with. If you want to be close to, but just after sunset most of the time, then adding 2 or 3 minutes to the HMNAO sunset times will be fine. If you want to be certain that sunset will be over, then you may want to add 5 or even 6 minutes.
I hope that helps — feel free to share this if you think it will be useful for others.
Prof Andy Newsam
Director of the National Schools’ Observatory
References & Acknowledgements
- Air Pollution and Sunlight Q&A ↩
- What Is Refraction of Light? ↩
- Atmospheric Turbulence ↩
- Effect of atmospheric refraction on the times of sunrise and sunset ↩
- Refraction near the Horizon ↩
- A note on sunrise, sunset and twilight times and on the illumination conditions during twilight. ↩
- Why can we see the sun’s image before sunrise and after sunset? ↩
- Variability in the Astronomical Refraction of the Rising and Setting Sun ↩
- Variability in low altitude astronomical refraction as a function of altitude ↩