THE SOUTHWORTH PLANETARIUM
207-780-4249       www.usm.maine.edu/planet
70 Falmouth Street  Portland, Maine 04103
43.6667° N,                    70.2667° W
Founded January 1970

                    "The  outside inside turned inside out ."

THE DAILY ASTRONOMER

Monday, March 21, 2016

Pandora Springs

Yesterday, spring sprung!

We celebrate spring's springing action with a deep dip into Pandora's Jar: the mythological query repository that we've tucked into the deepest planetarium crannies where even the shadow people are loath to venture.   We decided to collect a few questions pertaining to spring.  We even repeated a previous question about balancing eggs on the equinox.

Why is the vernal equinox called the "First Point of Aries" when the Sun is actually in Pisces on this date?

The Sun used to be "in" the constellation Aries on the first day of Spring, otherwise known as the vernal equinox.   We should explain that as Earth revolves around the Sun, the latter will appear to travel through thireen constellations comprising the "zodiac."  Greek astronomer Hipparchus of Nicea (190-120 BCE) introduced the term "First Point of Aries" (or "Cusp of Aries.") when he observed that the Sun was within the constellation Aries during the vernal equinox.     However, the Sun's apparent vernal equinox position has continously shifted along the ecliptic by about 1 degree every 73 years due to precessional wobbling.   The wobbling, caused primarily by interactions with the Sun and Moon, causes our planet's pole to describe a 47 degree circle through the sky every 26,000 years.   Consequently, the thirteen zodiac constellations will all "host" the vernal equinox point during this 26,000 year cycle.        According to astronomer Jean Meeus, the vernal equinox point crossed the Aries-Pisces border in 68 BCE.  Ironically, this shift occurred less than a century after Hipparchus' death. The vernal equinox point has been moving westward through Pisces ever since.  In AD 2597, the vernal equinox will move into Aquarius the Water Bearer.  Or, to be more specific, it will cross into the rectilinear region that the International Astronomical Union has designated as the Aqarius "region." Perhaps even then, astronomers will continue to refer to the vernal equinox as the "First Point of Aries."

Why isn't the day length on the first day of spring exactly 12 hours?

Simply because we're not living on a perfectly spherical, uniformly dense sphere devoid of an atmosphere.  First, Earth is an oblate spheroid, an object that looks spherical, but contains a "bulge" around its middle due both to its rotational motion and tidal stresses. Secondly,  land is not divided evenly between the Northern and Southern Hemispheres.   The Northern Hemisphere currently contains 68% of Earth's land mass.  The Southern Hemisphere contains thirty two percent.  This uneven land distribution has caused Earth to develop a slight "pear shape."  Consequently, sunlight falls unevenly on Earth.   Third, the atmosphere refracts light, which "pushes" the image of the Sun above the horizon, even when it is actually just under the horizon!

Interestingly (well, at least to us), the actual date when one experiences twelve hours of daylight and twelve hours of night varies with the latitude.    At Portland's latitude (43 degrees),  the parity between day and night occurs around March 17th and September 25th.    Observers at 60 degrees North experience this parity around March 18th and September 26th, while those at 20 degrees North have precisely twelve hours of daylight around March 14th and September 28th.      

Even though the word "equinox" is derived from the Latin term "aequinoctium," meaning "equal night," the length of daylight and night is not actually equal on the equinox.

The March Equinox marks the first day of spring in the Northern Hemisphere and the first day of autumn in the Southern Hemisphere.  What season will it be on the Equator?

The Equator is a line of no thickness and therefore doesn't qualify as an actual location.  Let's assume that you're standing on the Equator on the equinox. Or, more correctly, you're straddling the equator so that one  foot is to the north of it and the other is to the south of it.    At the moment of the equinox, your northern foot would be in spring and your southern foot would be in autumn.

Is it  true that you can balance eggs on the equinox?

Now, we could be difficult and merely say, "Yes," before snapping off the e-mail and being done with it. Even those, JR, who think we could do with a bit more brevity might consider that response insufficient. Therefore, we'll offer a more thorough explanation:

Yes, you can balance eggs on their ends on the equinox.

Of course, provided you have enough patience, you can balance eggs on their ends any day of the year. You'll have better luck if the egg ends are slightly flat, but the equinox, itself, has no effect.

The notion that a one can balance an egg on its end only on the equinox stems from the mistaken belief that Earth's tilt varies throughout the year: that it veers from an extreme (23.5 degrees) at either solstice to being upright (0 degree tilt) on each equinox. When perfectly vertical, Earth can keep everything on its surface in balance, even those objects such as eggs that would fall over on any other day.

Of course, Earth's tilt doesn't vary throughout the year.* It stays the same. What changes is the orientation of the poles relative to the Sun. On the summer (June) solstice, the north pole is aligned toward the Sun, whereas the south pole is directed away from it. Six months late, the north pole is oriented away from the Sun, but the south pole points toward it. The actual planet tilt remains constant. Imagine for a moment that you have a pen tilted over by a certain amount. Have it revolve around a stationary object, such as a ball. Notice that at one location, the top of the pen is pointing toward the central object, but the bottom is pointing away from it. At the opposite point, the top is pointing away, but the bottom is pointing toward the ball.

Today, one can attempt to balance an egg on its end. One might well succeed, provided one that one is patient enough. As I said, having a slightly flattened egg end is helpful. I hope this answer was helpful, as well.

*A little pedantic aside: Earth's tilt, or obliquity, does vary with time. Presently, the tilt is just about 23.5 degrees (23.44). By the year 4000, it will be less than 23 degrees, By 10,000 AD, the obliquity will be about 22.5 degrees. When regarded over the long term, the obliquity varies from a maximum of 24.5 degrees to 22.5 degrees, generally over a 41,000 year period.