Happy Solstice, and How To Find an NLC!

Today is the summer solstice for those of us in the Northern Hemisphere, and the winter solstice for those in the Southern Hemisphere.  So that means the sun was up quite early for me this morning, and will linger well into the evening — just as well it’s such a glorious day outside!  But spare a thought for our Muslim friends, who are observing Ramadan and who must keep their fast especially long today.

Which raises an interesting question: what do Muslims do if they live north of the Arctic Circle?  Well, most just adopt a Mecca clock for the duration, since it’s obviously not possible to go completely without food or drink for the entirety of Ramadan.  A lot of the refugees to Sweden have been placed in Kiruna, which is above the Arctic Circle and well into the land of the midnight sun.  The Sun rose there just after midnight on May 27.  Ramadan began June 5 — technically at sunset, but in Kiruna of course the Sun did not set that day.    Ramadan will end at sunset on July 5 — but in Kiruna, the Sun will stay up, wandering around the entire horizon, until it finally dips down for about half an hour on July 17.  Let’s just say folks up those latitudes invest in good blackout curtains.

That said, it isn’t *actually* the longest day.  Well, it *is*, but that’s not actually what a solstice is.  The longest day of the year is usually very close to the solstice, but sometimes falls either before or after it.  What’s really happening today is that the Sun is the farthest north it ever gets in the sky.  Well, north in an astronomical sense.  If you’re in Kiruna, or Barrow, Alaska, or Tromso, Norway, or any of a large number of other communities above the Arctic Circle, you are already painfully aware of a Sun hanging due north in the sky (at local midnight; it’s due south at local noon).  But if you could plot the Sun’s position against the background stars (which are very hard to see during the daylight, but are there all the same), you’d see it is as far away as it can get right now from the celestial equator — a line in the sky that you get by projecting the plane of the Earth’s equator out into space.  Not coincidentally, this is also the band occupied by the Zodiac.  The ancients used the constellations of the zodiac not just for astrology but also as a natural calendar.  This is why, for them, the solstices (and equinoxes) were so important.  It marked the passage of time with excellent precision — far better than you can get by counting lunar cycles.  Because while the Moon’s phase will shift regularly, it doesn’t shift an even number of times per year, but each constellation will take exactly (well, exactly enough…) one year to return to its original position in the sky.  So if you want to know when to plant crops, you remember which constellation is in the east at sunrise at the correct time of year.

Talking of which, by sheer happenstance, today is also full moon, so the bright day will be followed by a bright night.  Bummer if you wanted to look for noctilucent clouds — this is the peak time for seeing them.  If you want to try anyway, in hopes the lunar glare won’t wipe them out, try looking to the west about half an hour to an hour after sunset, for electric blue tendrils of cloud across the sky.  These clouds are too faint to be seen in daylight, but are so high altitude that they remain illuminated long after all other clouds have gone into shadow.  They’re considered space weather; they occur above the stratosphere, and you could think of them as the smoke trails of micrometeoroids, though they’re really water vapor.  (Water molecules find their way into the ionosphere, above the stratosphere, and interact with micrometeoroids, crystallizing around them.)  I’ve yet to catch a noctilucent cloud myself, but I’m sure hoping.  😉


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