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Starts With A Bang

This Is What’s Special About A Full Supermoon Occurring On The Equinox This Year

If you’re careful enough, you can learn more about the Solar System than you ever imagined.

This year, on March 20th, the full Moon will illuminate the night sky. With the Moon reaching perigee, or the point in its orbit where it comes closest to Earth, just the day before, we’ll have a supermoon: where the Moon appears substantially larger and brighter than average. Although supermoons happen a few times per year, the one occurring on March 20th is special.

That’s because it’s also the spring equinox, where the Earth’s axis is tilted perfectly perpendicular to the imaginary line connecting the Sun and Earth. While we get a spring equinox every year, the coincidence of a full Moon with the spring equinox is much rarer, as it happens only once every 19 years. This is a pretty special occurrence, and it gives us the opportunity to learn some things about our world that we rarely get to explore.

Just 800 years ago, perihelion and the winter solstice aligned. Due to the precession of Earth’s orbit, they are slowly drifting apart, completing a full cycle every 21,000 years. 5,000 years from now, the spring equinox and the Earth’s closest approach to the Sun will coincide. Note that, at all times, the tilt of the Earth during the equinox is perpendicular to the imaginary line made by connecting the Sun to the Earth. (GREG BENSON AT WIKIMEDIA COMMONS)

Although equinox literally means “equal night,” it isn’t exactly true that there are 12 hours of day and 12 hours of night everywhere on Earth during the equinox. Nor is the equinox defined by a specific calendar day; rather, it’s a specific moment in time that corresponds to planet Earth passing through a special point on its orbit.

If you were to draw an imaginary line through the Earth, from North Pole to South Pole, you’d find the line that represented Earth’s rotational axis. If you drew a second imaginary line, connecting the center of the Earth to the center of the Sun, you’d see that these two lines made an angle with one another. During the solstices, the Earth’s axis tilts maximally towards or away from the Sun. But during the moment of equinox, you make a right (90°) angle, which means that something very special occurs.

Even though the Earth always rotates on its axis, which is tilted at 23.4 degrees, the equinoxes are special for having that axial tilt be perpendicular to the Sun-Earth plane, rather than at an inclined angle, which occurs on all other days of the year. (LARRY MCNISH / RASC CALGARY CENTRE)

At the moment of equinox, all of the Sun’s rays falling on Earth are perpendicular to that imaginary line. This means that the following things are true at the two equinoxes, and not on any other day of the year:

  • Exactly 50% of the Sun is visible from the North and South poles. (If the Sun were a point, instead of a disk, it would be barely visible from both locations.)
  • From where the midpoint of the Sun rises over the horizon to where the midpoint of the Sun falls below the horizon takes 12 hours from everywhere on Earth’s surface.
  • And if you measure the shadow cast by a perfectly vertical stick when the Sun is at its zenith (highest point in the sky), the angle you measure will reveal your precise latitude.
As the Sun moves throughout the sky, there will be an instant where a shadow cast by a perfectly vertical object reaches its minimum length. If you can measure the angle cast by the shadow at that time, you’ll get an angle that corresponds perfectly to your latitude, but only if you perform this measurement on the day of the equinox. (BORA SHIN)

But it’s very rare indeed to get a full Moon that coincides with the equinox, because of a fact known for thousands of years: the lunar calendar (where you keep track of time based on the new-to-full-and-back-to-new Moon cycles) and the solar calendar (where you keep time based on the Earth’s orbit around the Sun) line up after 235 lunar months, or 19 calendar years.

This month, March of 2019, sees the full Moon and the equinox line up. It will happen again in 2038; it almost happened (it missed by a few hours) in 2000, and it happened before that in 1981. Every 19 years, we have this alignment. Similarly, the September equinox full Moons occurred in 1991, 2010, and will return in 2029.

On September 22, 2010, skywatchers were treated to the last equinox full Moon. The full Moon on March 20th/21st, 2019, will be the last one we’ll have until 2029. Here, the moon is seen rising behind Coit Tower in San Francisco, California. (GETTY)

You may remember that two months ago, in January, we received a total lunar eclipse: where the Sun, Earth, and full Moon were perfectly aligned. Now that it’s two months later, the full Moon is misaligned, because the plane that the Moon orbits the Earth in is tilted with respect to the Sun.

This is incredible for science! On any old equinox, you can measure your latitude on Earth; on any solstice, you can measure the axial tilt of the Earth. Well, on an equinox that coincides with a full Moon, you can measure how far out of the Sun-Earth plane the Moon actually is at this particular moment in time. And if you know when the last eclipse was and the next eclipse will be, you can actually determine the tilt of the Moon’s orbit. Here’s how.

Eclipses only occur when the nodes of the Moon’s orbit, where it crosses the Earth-Sun plane, line up with a new or full phase. This is completely different from where apogee and perigee are on the Moon’s elliptical orbit, but these also migrate throughout the course of a lunar year; the Moon’s orbit is perturbed by the other masses in the Solar System, like the Sun, in its orbit around Earth. (JAMES SCHOMBERT / UNIVERSITY OF OREGON)

The Moon orbits the Earth in an ellipse, and that ellipse is tilted with respect to the Earth-Sun orbital plane. That ellipse remains in a constant shape, however, as the Earth orbits the Sun. There are two points where the Earth-Moon plane crosses the Earth-Sun plane, known as the line of nodes. Every six months, the nodes approximately line up with the imaginary line connecting the Earth to the Sun, which enables eclipses to occur: solar eclipses if the near node is aligned, lunar eclipses if the far node is aligned.

An illustration of the Sun-Moon-Earth configuration setting up a total solar eclipse. When the Moon’s shadow falls on Earth when the nearer-to-the-Sun node aligns, we get a solar eclipse. Conversely, when the away-from-the-Sun node aligns with the Earth-Sun plane, the Moon passes through Earth’s shadow and gives us a lunar eclipse. The last eclipse, as of today, was January 20/21, 2019, which was a total lunar eclipse. (STARRY NIGHT EDUCATION SOFTWARE)

Three months (or, more precisely, 88 days) from perfect alignment, we’ll achieve maximum misalignment. Three months after a total solar eclipse, the new Moon will be misaligned from the Earth-Sun plane by the maximal amount; three months after a total lunar eclipse, the full Moon will be misaligned from the Earth-Sun plane by the maximal amount.

The equinox is special, because if you measure the smallest shadow cast by a vertical stick during the day, you can measure a particular angle to the shadow, which tells you your latitude.

If you construct an apparatus with a perfectly vertical stick, you can use the moment at which the shadow is shortest to identify the angle at which the Sun (or Moon) must be at the zenith. For the Sun on the equinox, we can calculate our latitude; for the Moon, as long as we know our latitude, we can devise a way to calculate the Moon’s orbital tilt. (LARRY SESSIONS / COMMUNITY COLLEGE OF AURORA)

If you then performed that same exact experiment at night, by moonlight, you’d get a different particular angle that’s extremely important. If you took that measurement you got for your angle by moonlight, and subtracted your latitude (or your daytime-measured angle), you’d get your measured angular misalignment for the Moon, at present, relative to the Earth-Sun plane.

All that’s left to do, in order to know the tilt of the Moon’s orbit, is a little math.

The Moon, although far less bright than the Sun, is quite capable of casting shadows with the light that it gives off, particularly during a full Moon. If you measure the shadow cast by a vertical stick, similar to how you’d do it for the Sun, you’d find a different angle, which can help you reveal the Moon’s orbital inclination around the Earth-Sun system.(MACIEJ KRAUS / FLICKR)

Because it takes approximately 88 days to go from maximum alignment of the Moon’s orbit to maximum misalignment, then knowing that there was a total lunar eclipse on the evening of January 20th/morning of January 21st and that the equinox is on the evening of March 20th (or the morning of March 21st) allows you to do that key calculation.

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There are 59 days between January 21st and March 21st, but it takes 88 days to go from maximum alignment to maximum misalignment. So take that measured angular misalignment, divide it by the sin((59/88)*90°) that occurred from the prior lunar eclipse until today’s full Moon, and that’s the tilt of the Moon’s orbit!

When the Earth’s north pole is maximally tilted away from the Sun, it’s maximally tilted towards the full Moon, on the opposite side of the Earth. The Moon stabilizes our orbit but also slows the Earth’s rotation. The total inclination of the Moon’s orbit, of 5.1°, won’t be achieved during the equinox, but a mathematical look back at the last eclipse will help us calculate the total tilt. (NATIONAL ASTRONOMICAL OBSERVATORY ROZHEN)

This is a case where we actually know what the answer is, and so with just a little math-in-reverse, we can predict what we ought to observe on the night of March 20th/21st. Since the Moon’s known orbital tilt with respect to the Earth-Sun plane is 5.14°, we can multiply it by the math factor we just devised: sin((59/88)*90°), which comes out to about 5.1°*0.87, or about 4.4°.

No matter where you are on Earth, so long as you can see the Sun and the Moon clearly when they’re at their highest points in the sky and there are no nearby lights ruining your shadows, you can perform this measurement for yourself. Sure, it’s rare that we get a full Moon during the equinox (about once per decade), and even rarer (about a 1-in-4 shot) that the full Moon is a supermoon, but the science you can do is the real star of the show. After all, how often do you get to figure out the Solar System for yourself?

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.