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Home >Opinion >Columns >Here comes the sun; I say it’s all right

Here comes the sun; I say it’s all right

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The 23.5 degree tilt of the Earth’s axis actually affects the length of the solar day

Since about the start of November, I’ve been keeping a daily log. This was prompted by a phenomenon I first heard of years ago, when a friend and mentor drew my attention to it. But this winter season, I decided to keep a log just so I could see it unfold, in a manner of speaking, on the page. Sure enough, unfold it has.

Since about the start of November, I’ve been keeping a daily log. This was prompted by a phenomenon I first heard of years ago, when a friend and mentor drew my attention to it. But this winter season, I decided to keep a log just so I could see it unfold, in a manner of speaking, on the page. Sure enough, unfold it has.

What’s in the log? For each day, I note sunrise and sunset times, the consequent span of daylight that day, and the actual time when it was noon. I expect you have a furrowed brow over that mention of “noon". “Dilip," you’ll yell shortly, “haven’t you heard of 12 noon?" But let me come back to that.

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What’s in the log? For each day, I note sunrise and sunset times, the consequent span of daylight that day, and the actual time when it was noon. I expect you have a furrowed brow over that mention of “noon". “Dilip," you’ll yell shortly, “haven’t you heard of 12 noon?" But let me come back to that.

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We all know that 21 December is one of two solstices in the year. In the northern hemisphere, it’s the winter solstice; south of the equator it’s the summer solstice. What that means, we also know: it is the shortest day of the year in the northern hemisphere and the longest, south of the equator—and for the rest of this column, I’ll stick to a northern hemisphere perspective.

Of course, even the shortest day still has the same 24 hours (with a caveat I’ll also come back to). What “shortest" really means here is that on 21 December, we have daylight for the shortest time in the year. That is, the span from sunrise to sunset is the year’s shortest on that day. Correspondingly, the longest night of the year occurs on 21 December.

You understand my log now. I’m looking at it, and right enough, the period of daylight decreased steadily from about 11 hours and 18 minutes on 10 November to 10:59 on 21 December. Then, it increased steadily to about 11:06 yesterday, 14 January. No doubt you also know that it will keep increasing for nearly six more months. On the spring equinox, 20 March, day and night will be equally long; three months later, we’ll have the longest day (and thus the shortest night) of the year on the summer solstice, 21 June.

So far, straightforward stuff. Now you might expect that the increase in daylight hours after 21 December is happening because sunrise and sunset are earlier and later, respectively, every day. That’s partly right: the sun has been setting a little later each day. On 21 December, it set at 6:06pm. On 14 January, it set at 6:20pm. Over three weeks, then, a slide of 14 minutes.

But—and this is why I really wanted to keep this log—the sun has been rising a little later each day too. If that’s counter-intuitive, it’s also true. The sun rose at 7:07am on 21 December; it rose at 7:14am on 14 January. Notice though that that’s only a 7-minute slide, compared to sunset’s 14 minutes in the same period. So it’s because sunset gets later at a faster pace than sunrise that we see ever-longer days after the winter solstice.

But 14 January is the last day of that trend, the day with the latest sunrise of the year. Starting today, the sun will rise earlier every day until June. That is, we’ll now have both earlier sunrises and later sunsets every day.

Just what is happening here? In fact, if you think about it, you’ll probably realize that there must be a mirror-image phenomenon that happens in the days leading up to the solstice. Sure enough, the earliest sunset of the year does not occur on the winter solstice. Instead, it’s on or about 27th November.

So yes, what’s happening? Why does the sun rise later every day for these weeks—about 24 days —after the solstice, while the day gets longer anyway? Is there some connection to the 23.5 degree tilt of the Earth’s axis?

There’s something to that speculation, but again, let me come back to that. First, remember that mention of noon above? What can the “actual time when it was noon" possibly mean? Isn’t that just noon, meaning that time in the middle of the day when the clock strikes twelve?

Actually, no. Noon is that time in the middle of the day when the sun is directly overhead—or strictly, at its highest position in the sky; when it is exactly halfway in its “journey" across the sky from sunrise to sunset. We call that noon, but it’s more than likely that the sun is not directly overhead when the time is 12 noon.

To understand that, remember, for example, that one time zone covers all of India, and so 12 noon is 12 noon whether you’re in Bhuj or in Aizawl. But at the moment the sun is at its highest in Aizawl, it’s not quite there in Bhuj. There is a difference, in other words, between “clock noon" (12 on the clock) and “solar noon" (when the sun is at its highest). On 14 January in Mumbai, for example, solar noon happened at 12:47pm. (In Bhuj, 1pm. In Aizawl, 11:28am).

So, let’s do this exercise. Since solar noon is exactly halfway through the day, call that 12 o’clock for now and divide the length of a given day by two. Subtract the result from 12 to get the sunrise time that day; add it to 12 for sunset. Call this “solar time", as opposed to clock time.

Using my log, I did this for each day starting on 21 December. A moment’s reflection will tell you that since the length of the day has increased every day since then, this exercise will give us steadily earlier solar time sunrises and steadily later sunsets. Which means the phenomenon that got me interested in this whole subject in the first place—the sun rising later every day between 21 December and 14 January, and only then turning around—disappears. Which is exactly right. On 21 December, the sun rose at about 15 seconds past 6:30am, solar time. On 14 January, it rose over three minutes earlier, at about 15 seconds before 6:27am, solar time. The solar time sunsets were correspondingly later through those days.

Which raises the question, why not do this subtraction and addition from the clock time for solar noon—for example, 12:47pm in Mumbai on 14 January? In fact, doing this for 14 January gives us precisely the sunrise and sunset clock times for that day. The thing is, though, that solar noon’s clock time varies too. In Mumbai, it was at 12:36 on 21 December, and has got steadily later in the days since, to 12:47 on 14 January. This progression, finally, is the reason for the steadily later clock sunrises till 14 January. After that, because the progression slows, sunrises start happening earlier by the clock.

This still leaves the question of why solar noon varies at all. Briefly: It’s at its earliest in November (about 12:22), gets later till about mid-February (12:52), gets earlier till mid-May and repeats this six-monthly cycle. This is because the tilt of the Earth’s axis actually affects the length of the solar day—the time from one solar noon to the next. It’s slightly more than 24 hours long around the winter and summer solstices, slightly less than that around the equinoxes (the caveat I mentioned somewhere above).

More about this in a future column. For now, I’ll leave you with this question: what, if anything, happens to sunrises and sunsets around the summer solstice, 21 June?

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun

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