Seeking the Summer Solstice

A summer solstice sunset. Image credit and copyright: Sarah and Simon Fisher.

Can you feel the heat? If you find yourself north of the equator, astronomical summer kicks off today with the arrival of the summer solstice. In the southern hemisphere, the reverse is true, as today’s solstice marks the start of winter.

Thank our wacky seasons, and the 23.4 degree tilt of the Earth’s axis for the variation in insolation. Today, all along the Tropic of Cancer at latitude 23.4 degrees north, folks will experience what’s known as Lahiana Noon, as the Sun passes through the zenith directly overhead. Eratosthenes first noted this phenomena in 3rd century BC from an account in the town of Syene (modern day Aswan), 925 kilometers to the south of Alexandria, Egypt. The account mentioned how, at noon on the day of the solstice, the Sun shined straight down a local well, and cast no shadows. He went on to correctly deduce that the differing shadow angles between the two locales is due to the curvature of the Earth, and went on to calculate the curvature of the planet for good measure. Not a bad bit of reasoning, for an experiment that you can do today.

Eratosthenes' classic experiment. Wikimedia Commons image in the Public Domain.
Eratosthenes’ classic experiment. Wikimedia Commons image in the Public Domain.

And although we call it the Tropic of Cancer, and the astrological sign of the Crab begins today as the Sun passes 90 degrees longitude along the ecliptic plane as seen from Earth, the Sun now actually sits in the astronomical constellation of Taurus on the June northward solstice. Thank precession; live out a normal 72 year human life span, and the solstice will move one degree along the ecliptic—stick around about 26,000 years, and it will complete one circuit of the zodiac. That’s something that your astrologer won’t tell you.

The tilt of the Earth's axis during the June northward solstice. Image credit: NASA.
The tilt of the Earth’s axis during the June northward solstice. Image credit: NASA.

The solstice in the early 21st century actually falls on June 20th, thanks to the ‘reset’ the Gregorian calendar received in 2000 from the addition of a century year leap day. The actual moment the Sun reaches its northernmost declination today and slowly reverses its apparent motion is 22:34 Universal Time (UT).  In 2016, the Moon reaches Full just 11 hours to the solstice. The last time a Full Moon fell within 24 hours of a solstice was December 2010, and we had a total lunar eclipse to boot. Such a coincidence won’t occur again until December 2018. You get a good study in celestial mechanics 101 tonight, as the Full Moon rises opposite to the setting Sun. The Moon occupies the southern region of the sky where the Sun will reside this December during the other solstice, when the Full Moon will then ride high in the night sky, and gets ever higher as we head towards a Major Lunar Standstill in 2025.

Image credit: Dave Dickinson
The back alley of our Morocco Air BnB mimics Eratosthenes’ well. Image credit: Dave Dickinson

Of course, this motion of the Sun through the year is all an illusion from our terrestrial biased viewpoint. We’re actually racing around the Sun to the tune of 30 kilometers per second. You wouldn’t know it as summer heats up in the northern hemisphere, but we’re headed towards aphelion or the farthest point from the Sun for the Earth on July 4th at 152 million kilometers or 1.017 astronomical units (AU) distant. And the latest sunset as seen from latitude 40 degrees actually occurs on June 27th at 7:33 PM (not accounting for Daylight Saving Time) go much further north (like the Canadian Maritimes or the UK) and true astronomical darkness never occurs in late June.

And speaking of the Sun, we’re wrapping up the end of the 11 year solar cycle this year… and there are hints that we may be in for another profound solar minimum similar to 2009. We’ve already had a brief spotless stretch last month, and some solar astronomers have predicted that solar cycle #25 may be absent all together. This means a subsidence in aurorae, and an uncharacteristically blank Sol.

But don’t despair and pack it in for the summer. As a consolation prize, high northern latitudes have in recent years played host to electric blue noctilucent clouds near the June solstice. Also, the International Space Station enters a second period of full illumination through the entire length of its orbit from July 26th to 28th, making for the possibility of seeing multiple passes in a single night.

A display of noctilucent clouds over Blackrod, UK. Image credit and copyright: Dave Walker.
A display of noctilucent clouds over Blackrod, UK. Image credit and copyright: Dave Walker.

And folks in the Islamic world (and travelers such as ourselves currently in Morocco) can rejoice, as the Full Moon means that we’re half way through the fasting lunar month Ramadan. This is an especially tough one, as Ramadan 2016 goes right through the summer solstice, making for only a brief six hour span to break the fast each  night and prepare for another 18 hour long stretch… and to repeat this pattern for 29 days straight. It’s a fascinating time of night markets and celebration, but for travelers, it also means odd opening hours and delays.

Searching for the solstice and other strange astronomical alignments at M'Soura, Morocco. Image credit: Dave Dickinson
Searching for the solstice and other strange astronomical alignments at M’Soura, Morocco. Image credit: Dave Dickinson

See any curious solstice shadow alignments in your neighborhood today?

Happy Lahiana Noon… from here on out, northern viewers slowly start to take back the night!


Getting Ready For International Space Station Observing Season

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The summer season means long days and short nights, as observers in the northern hemisphere must stay up later each evening waiting for darkness to fall. It also means that the best season to spot that orbital outpost of humanity—the International Space Station—is almost upon us. Get set for multiple passes a night for observers based in mid- to high- northern latitudes, starting this week.

This phenomenon is the result of the station’s steep 52 degree inclination orbit. This means that near either solstice, the ISS spends a span of several days in permanent illumination. Multiple sightings favor the southern hemisphere around the December solstice and the northern hemisphere right around the upcoming June solstice.

Here’s a rundown of the ‘ISS all night’ season for 2015. The Sun rises on the ISS after a brief three minute orbital night on May 30th, 2015 at 16:43 UT, and doesn’t set again until five days later on June 4th at 4:57 UT over the central US. The ISS full illumination season comes a bit early this year—a few weeks before the June 21st northward solstice—and the next prospect at the end of July sees the Sun angle juuust shy of actually creating a second summer season.

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The orbital trace of the ISS starting on May 30th. Image credit: Orbitron

NASA engineers refer to this period as high solar beta angle season. For a satellite in low Earth orbit, the beta angle describes the angle between its orbital plane and the relative direction of the Sun. Beta angle governs the satellite’s length of time in darkness and daylight. In the shuttle era, the Space Shuttle could not approach the ISS during these ‘beta cutout’ times, and the station generally goes into ‘rotisserie mode,’ as the ISS is rotated and its solar panels feathered to create alternating regions of artificial darkness in an effort to combat the continuous heating.

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A depiction of the beta-angle of a satellite.  Image credit: Fomirax/Wikimedia Commons

Why the 52 degree inclination orbit for the station? This allows the ISS to be accessible from launch sites worldwide in the spirit of international cooperation exemplified by the ISS. The station can and has been reached by cargo and human crews launching from Cape Canaveral and the Kennedy Space Center in Florida, the Baikonur Cosmodrome, the Tanegashima space port in Japan, and Kourou space center in French Guiana.

Our friend @OzoneVibe on Twitter suggested to us a few years back that a one night marathon session of ISS sightings be known as a FISSION, which stands for Four/Five ISS sightings In One Night. The prospective latitudes to carry out this feat run from 45 to 55 degrees north, which corresponds with northern Europe, the United Kingdom, and the region just north and south of the U.S./Canadian border.

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An amazing sequence showing a complete ISS pass overhead. Image credit and copyright: Alan Dyer/Amazing Sky Photography

At 72.8 by 108.5 metres in size and orbiting the Earth once every 92 minutes at an average 400 kilometres in altitude, the ISS is the brightest object in low Earth orbit, and reaches magnitude -2 in brightness—not quite as bright as Venus at maximum brightness—on a good overhead pass. Depending on the approach angle, I can just make out a bit of detail when the ISS is near the zenith, looking like either a box, a close double star, or a tiny Star Wars TIE fighter through binoculars. Numerous apps and platforms exist to predict ISS passes based on location, though our favorite is still the venerable Heavens-Above. It’s strange to think, we were using Heavens-Above to chase Mir back in the late 1990s!

There’s another interesting challenge, which, to our knowledge, has never been captured as we near high beta angle season for the ISS: catching an ‘ISS wink out,’ or that brief sunset followed by sunrise a few minutes later on the same pass. It’s worth noting that the central United States may see just such an event during an early morning pass on June 4th… will you be the first to witness it?

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An ISS pass over Denmark, Maine. Image credit: David Dickinson

Photographing the ISS is as easy as setting a DSLR on a tripod with a wide field of view lens, and doing a simple time exposure as it drifts by. Be sure to manually set the focus before the pass… Venus is currently well placed as a ‘mock ISS’ to get a fix on beforehand.

And amateur observers can even capture detail on the ISS, though this requires a camera running video coupled to a telescope. High precession tracking is desirable, though not mandatory: we’ve actually got descent results manually aiming a scope at the ISS with video running. The ISS appears in post production, occasionally skipping through the field of view.

PhD student Bob Lansdorp has made some great videos of the ISS with a similar rig.

Another unique method is to know when the ISS will transit the Sun, Moon or near a bright planet or star, aim your rig at the right spot, and let the station come to you. A good site to tailor alerts for such occurrences is CALSky.

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The ISS transits the Sun in 2012. Image credit and copyright: Fred Locklear

After high beta angle season, missions to and from the ISS will resume. This includes the return of ISS crewmembers Shkaplerov, Christoforetti and Virts on June 7th, followed by a Soyuz launch with Kononeko, Yui, and Lindgren on July 24th. Also on tap is SpaceX’s Dragon capsule on CRS-7 launching on June 26th, the return to flight for Progress on July 3rd, and a HTV-5 launch for JAXA on August 17th. These can also provide interesting views for ground observers as well, as these spacecraft follow the ISS in its orbit on approach like tiny fainter ‘stars.’

A busy season indeed. Don’t miss a chance to see the ISS coming to a sky near you, and watch as humans work together aboard this orbiting science platform in space.