Wednesday, January 4, 2012

Change in the time of sunrise throughout the year

I heard someone announce the other day that now that we are past the date of the winter solstice, the length of the days will increase by one minute every day until we get to the summer solstice.  Today is the 4th of January, 2012.  According to the Environment Canada Weatheroffice website, the sun rose at 8:48 a.m.  So will the sun rise tomorrow roughly one minute later than it did today?  The facts are available for checking as these kinds of astronomical observations are calculable and detailed lists of all kinds of related data are readily available on the internet from reputable sources.  What do these sources say?

The sun rose today in Thunder Bay at 8:48 a.m.  It will rise at the same time for the next three days.  From the 8th until the 10th it will rise earlier, at 8:47 a.m.  From the 11th to the 12th it will rise at 8:46 a.m., and so on.  By January 31st the sun will be rising at 8:28 a.m.  That's 20 minutes earlier than it rose today and 27 days from today.  So at the moment the sun must be rising less than one minute earlier each day.  If you look at a list of sunrise times for January you can see that the rate at which the sun is rising earlier is increasing.  That is, the sun is rising earlier faster.  While at the start of January it might take four days for the sun to rise a minute earlier, towards the end of January it is rising a minute earlier every day.

On the 1st of February the sun will rise at 8:26 a.m. while on the 29th of February it will rise at 7:39 a.m.  So from the first day to the last day of February, 28 days later, the sun rises 47 minutes earlier.  That is more than one minute earlier for every day of the month, on average.  In January it was less than one minute earlier for every day of the month, on average.  By looking at the list of sunrise times over the course of the month you can also see that the trend is one of the sun rising earlier faster.  That is, at the start of the month the sun is rising roughly a minute earlier each day but by the end of the month it it rising by roughly two minutes earlier each day.  When will the rate at which the sun is rising earlier stop increasing and start decreasing?

In March the sun rises earlier by two minutes almost every day.  But if you find a list that also shows the length of the days, something significant happens in March.  At the start of March the length of the days is increasing by 3 minutes and 30 seconds each day.  By the middle of the month day-length is increasing by 3 minutes and 33 seconds.  The increases in day-length have been getting larger since the winter solstice.  On December 22nd of 2011 the length of the days stopped decreasing and started to increase as we passed the shortest day of the year.  Sunrise and sunset times follow this trend: at around the shortest day of the year, the sunrise time stops getting later each day and begins to get earlier; the time of sunset stops getting earlier and starts to get later; the length of the day stops shrinking and starts expanding as the sunrise and sunset times move away from each other leaving a longer period of sunlight between them; the sun is in the sky for longer each day.  Back to March and the rate at which the time of sunrise is getting earlier and earlier.  In the middle of the month the sunrise is getting earlier and earlier by about 3 minutes and 33 seconds each day but then it starts to fall, rising 3 minutes and 32 seconds earlier each day.  By the end of March the sun is rising 3 minutes and 31 seconds earlier each day.

By the end of April the sun is rising 3 minutes and 7 seconds earlier each day.  Sunrise is still earlier each day.  On the 30th of April the sun will rise at 5:39 a.m. (6:39 a.m. if you put your clock forward for daylight savings).  The latest sunrise occurred close to the winter solstice, on the 1st of January, at 8:49 a.m.  By the end of April it is more than three hours earlier.  The time of sunrise will continue to get earlier but the rate at which this will happen will slow down until the middle of June when the sun will begin to rise later each day and the length of the days will start to decrease again.  The summer solstice will occur on the 21st of June when we will experience the longest day of the year.

Although the winter solstice, the shortest day of the year, occurred on December 22nd, the latest sunrise occurred on January 1st, only four days ago.  However, the earliest sunset occurred about a week before the solstice.  So while the sun was rising later and later up until a few days ago, it has been setting later for the last two weeks.  The sun does not start rising earlier on the same day that it starts to set later.  There is a gap of about two weeks where, in the winter, the sun is rising and setting later, and then in the summer, it will be rising and setting earlier.  I don't know the reason for this but our clock is an artificial linear measurement superimposed on a natural diurnal (means daily) cycle that varies and wobbles in all sorts of complicated ways.

Sunset in Bobcaygeon, Ontario, in December 2011


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Ms.April said...

That photo is unbelievable! Nature is amazing!

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