Today Is the Winter Solstice – Here's What That Means

What’s A Solstice?

For many of us, December 21, 2016, marks this year’s Winter Solstice. But for plenty of others, it’s also the Summer Solstice. How can this be? What is actually unique about today from an astronomical sense? What is a solstice anyhow?

First, let’s be considerate of all people regardless of whether they live in the northern or southern hemisphere. Let’s refer to today’s event as the December solstice. We do this because while those of us that live in the northern hemisphere consider it to be Winter, our friends south of the equator are in the middle of their Summer. This, of course, is because the Earth is tilted on its axis–which means that for part of the year, the northern part of our planet is tilted towards the Sun (and thus receiving more exposure to solar radiation i.e. Summer), while the southern half is tilted away (less solar radiation i.e. Winter). The other half of the year, this is reversed. (If you live directly on the equator, you can call it whatever you want because you get to have Summer 365 days a year.)

A solstice occurs on the day when the planet’s hemisphere has its maximum (or minimum) amount of solar exposure (in December, it’s the northern; in June it’s the southern’s turn), which in turn gives us the longest and shortest days of the year. Contrast that to the equinoctes (yes, that really is the plural form of equinox), in which the length of day is equal to the length of night.

Now, you might have heard the solstices being referred to as the “First Day of Spring/Winter”, which if you live in a geographic area that has noticeable seasons you know this isn’t true. It would be more appropriate to consider a solstice as the middle of a season. Starting today, the daylight hours get longer in the northern hemisphere and shorter in the southern.

So where did the “First Day of Spring/Winter” idea come from?


I really don’t know.

If you find out, please let me know.

Happy March Equinox: An Explanation

March Equinox graphic

Solstice and equinox diagram, showing the March equinox

Image Credit: NASA

Today is the March Equinox. You’ve probably already heard it a few times today; people running around proclaiming with utmost exuberance how today is the first day of Spring. After the long winters that some of us endure, the arrival of Spring is welcome news. But what is really going on today? After all, where I live it still feels like the middle of Winter, but flowers were already blooming on a trip I took to California a couple of weeks ago1. If we based “The First Day of Spring” on climate patterns, regions across the globe would be recognizing a wide variety of days throughout the year.

When someone says today is the first day of Spring, what they really mean (whether they know it or not) is that today represents an equinox; specifically, the March Equinox.2 On Earth, an equinox is the point in its orbit around the Sun when both hemispheres are equally illuminated; our tilted Earth lines up to a point in which the Sun passes directly over the equator. This happens twice a year, on the March and September equinoctes (which I learned today is the proper plural form of the word equinox).

Contrary to popular belief, the day of the equinox does not represent the day where daylight and darkness are equal. You can thank geometry, the atmosphere, and the Sun’s angular diameter to cause that equality to happen at different times geographically. What today does mean though, is that the equinoctes are the only two days in which the Sun rises due-East and sets due-West, and which the Sun would pass directly overhead from an observer on the equator.

One other very important thing that you must know if you don’t learn anything else today. Way too many people believe that the equinoctes are the only day of the year that an egg can be balanced on its end. While its true that on the equinox an egg can be balanced, it’s also true of every other day of the year; it makes no difference!

There are other times during the year (read: our orbit around the Sun) that we recognize Earth residing at a special place.  There’s Perihelion (which we went over in January) and Aphelion, and then the widely-celebrated solstices; but I’ll save that for another time.

Happy March Equinox!

This article originally posted on March 20, 2012.

  1. In fact, while it may have still been Winter to the San Diegans giving me quizzical looks for swimming in the ocean without a wet suit, to an Alaskan like myself it felt like an unusually warm Summer’s day!
  2. What about them being called the Spring  and Fall (or their Latin names, Vernal and Autumnal) equinoctes? Well, that wasn’t exactly fair to those in the Southern Hemisphere, whose seasons are opposite those in the Northern Hemisphere.

With The Supermoon Behind Us

Supermoon sinking into the atmosphere.
(International Space Station crewmember, André Kuipers, snapped this photo of the Supermoon sinking behind the Earth’s atmosphere.)
[Image credit: André Kuipers]

Did you get a chance to see this year’s “Supermoon“? Still confused as to what was so super about it, anyhow? Simply, the supermoon is the colloquial name for what is scientifically referred to as the perigee-syzygy moon. “The … what”, you ask? Don’t worry, it’s not as complicated as it sounds. The Moon orbits our Earth, not in a perfect circle, but in an ellipse. As a consequence of this, there are times the Moon is closer to the Earth and times it is further away. For any object orbiting the Earth, the part of its orbit that takes it furthest from our planet is called apogee. The closest point, perigee.

So now that we have perigee out of the way, “what was that other funny word again?” A syzygy,  (pronounced, Sizz-ih-gee), is a term used to refer to an astronomical event in which 3 celestial bodies form a straight line. In our case with the Moon, the bodies are the Sun, the Earth, and the Moon. You’re probably realizing that the Sun-Earth-Moon system experiences two syzygies each month; we call them the New Moon and the Full Moon.  The lunar month (29.53059 days) is defined as the period of time between two identical syzygies (Full Moon-to-Full Moon / New Moon-to-New Moon).

Putting it all together now: a perigee-syzygy Moon is the Full Moon or New Moon which coincides with its closest approach to Earth. Keep in mind, a New Moon at perigee could also be referred to as a supermoon; however, it’s unlikely to generate much attention because we can’t see the New Moon from Earth. “Well, of course. That makes sense!

So now that we know what a supermoon perigee-syzygy Moon is, let’s talk about what a perigee-syzygy Moon does; or, more importantly, doesn’t do. There is no correlation between perigee and major earthquake activity. There is certainly no correlation between perigee and human behaviour (well, except for the fact that when people start talking about supermoons, more people are likely to take a look at the Moon on that occasion). “But what about bigger tides?” Well, yes! Tides are greatest during Full and New Moons, and there is an increase in the tides when the Moon is closer to the Earth as well. Luckily, tidal forces are weak and even the few percent increase due to the perigee-syzygy isn’t going to create anything that will cause alarm.

But I heard the supermoon is super big and super bright!” While the perigee-syzygy Full Moon is what we can call the biggest and brightest Moon of the year, it’s such a small degree bigger and brighter that its really not noticeable. In fact, last night’s supermoon was only about 1% bigger/brighter than last month’s Full Moon. It did appear 14% larger than the smallest Moon of the year, but again, you’d have to be using some tools other than just your eyes to notice the difference.

An image showing the difference between perigee and apogee Full Moons.
(This image shows the difference in apparent size between a Full Moon at perigee and a Full Moon at apogee. Lined up next to each other, the difference looks quite large. In the sky by themselves, you’d be hard-pressed to notice the difference.)
[Image credit: Copyright © 2001-2012, Anthony Ayiomamitis]

Now, I purposely waited until after the Supermoon had passed to offer this explanation. Why? Because I didn’t want to discourage people from thinking they might see something special if they looked up at the Full Moon last night. It wasn’t easy to stay quiet for a couple of reasons. First of all, all of the ridiculous claims and fear that is generally associated with this event is hard to ignore — and in cases where real fear was involved, I did explain how there was nothing to worry about. The other reason it was difficult to not publish this before the event was that I didn’t want to entirely erase the hype that inevitably surrounds the “Supermoon”. Call it selfish, but I wanted people looking up at the sky last night — even if it was under some slight false pretenses. I want people looking up every night, and if some buzz on the internet can help make that happen, well then… good.

The truth is, the Moon is amazing whenever you can see it. The light of a Full Moon creates amazing shadows on our planet, and is a comforting companion to have overhead at night. Waxing and waning Moons are also beautiful, because they occur at an angle with the Sun in which the shadows and craters are much more pronounced.  And a New Moon (one we cannot see) offers us the darkest skies to observe the other billions of fascinating objects that are just above our heads.  All of which are… well… Super.

ATREX – Observing the Air Up There

I suspect we might hear about strange sky phenomena and UFOs occurring over the US Eastern Seaboard tomorrow, thanks to NASA’s ATREX mission.
After previously being scrubbed, the next launch attempt has been set for the wee hours (2am – 5am EST, or what I might consider late tonight) of March 27. ATREX, or the Anomalous Transport Rocket Experiment, is designed to study ultra-high altitude, high-speed wind patterns that have been observed on the very edge of space1. Data suggests that 200 – 300 mile-per-hour winds occur at an altitude of 62 – 68 miles; though little is yet understood about the phenomena. The atmosphere at that height is incredibly thin, and it essentially takes a rocket to get there.

The project will complete its test with the use of five of what are referred to as sounding rockets, launched within minutes of each other. These sounding rockets are smaller than those that are used to achieve orbit or carry heavier payloads, but will work just fine for this experiment. After reaching an altitude of 50 miles, the rockets will release a chemical tracer that will be observed from camera facilities both North (New Jersey) and South (North Carolina) of the Wollops Flight Facility in Virginia. The chemical, trimethylaluminium, was selected due to its reaction to oxygen; it glows and produces aluminium dioxide, carbon dioxide, and water vapor (each already present in our atmosphere).

Diagram of ATREX mission
(Graphic showing various aspects of the ATREX mission. Click for larger version.)
[Credit: NASA/Goddard Space Flight Center]

If you’re not on the US East Coast, but still want to try and watch the show, NASA has a webcast available here: and a UStream will carry it here:

For more information, you can follow the Wollops Flight Facility on Twitter, and check out the video below.

  1. For an important point of clarification, when I use the term “edge of space”, I mean it literally. You will often hear about an amateur balloon going to space, or a sky-dive from or near space, but those are exaggerations in my opinion. In my book, we’re talking at least 50 miles (the point at which NASA gives you astronaut wings) or better yet, the Kármán line.

Sunday Matinee – Aurora Borealis and North America at Night

[Video Credit: Image Science & Analysis Laboratory, NASA Johnson Space Center]

This video was taken by the crew of Expedition 29 on board the International Space Station. The sequence of shots was taken October 18, 2011 from 07:09:06 to 07:27:42 GMT[.]

This video compacts about 18-and-a-half minutes of ISS travel into about 30 seconds.

What I particularly enjoy about this video is that it starts looking directly to the area of the planet I occupy. Alaska’s Kenai Peninsula is easily distinguishable, especially due to the city lights of Kenai, Soldotna, Homer, and Seward. South of that you can see Kodiak Island. Immediately north of the Kenai Peninsula, Anchorage is brightly lit; Fairbanks can be seen even further to the north. The video travels east across the United States, as you find yourself struggling to decide on whether you should watch the aurora borealis (northern lights) to the north, or try to guess all of the cities you can see due to the concentrated man-made light. Clouds obscure much of the view between Alaska and the Rockies, but they break in time to offer great Canadian views of Calgary and Edmonton.

Following that, Minneapolis/St.Paul stand out, just before Chicago takes center-stage at around 24 seconds in, brilliantly lit, just south of Lake Michigan. You can catch the flashes of a lightning storm as we continue east towards the East Coast of the United States. You might notice how populated and electrified the East Coast is, compared to the Mid-West and the central United States. The video ends just after our view heads east over the coast of Florida, and above the shallow waters of the Bahamas.