Copyright Tau’olunga, Wikipedia. This file is licensed under the Creative Commons Attribution ShareAlike 2.5 License.
Do you know what causes the seasons?
It’s a lesson we learn in grade school and usually forget. And that’s unsurprising, with an answer so counterintuitive.
It’s the tilt!
If the Earth weren’t tilted on its axis, the temperature would remain fairly constant throughout the year. Some extra warmth would arise in January, with the Earth closest to the Sun. And the Northern Hemisphere in general would enjoy milder weather than the Southern, because land masses (concentrated up north) retain heat better than water does. But the variation would be nothing like what we experience now.
The Earth is continually pointed in the same direction as it makes its rounds, 23.5 degrees from the vertical (i.e., from a line perpendicular to the Earth’s orbit). At one point in space and time in the Earth’s travel around the Sun, the Northern Hemisphere points towards that body. This time is the northern summer solstice, normally occurring June 21 in many places (including Greenwich, UK, the point from which time is measured) but—there being nominally 24 time zones on our planet, and in practice, even more—not all.
The OrbiTee shows the Earth on June 21, the date during which Denver, Repulse Bay, and New York normally experience their summer solstice. But at the moment shown here, it is no longer June 21 in Crystal Sound, the time of which is driven by Greenwich Mean Time/Universal Coordinated Time.
This is why we don’t simply label the Earth “June 21,” as in other visual explanations of the seasons, and leave it at that.
At or around this same date, Sao Paulo, Crystal Sound, and the rest of the Southern Hemisphere experience their winter solstice, because this Hemisphere points away from the Sun.
At more or less the opposite point in the Earth’s orbit, the Northern Hemisphere points away from the Sun. This is the northern winter solstice, normally occurring December 21 in many places, including Chicago and Paris, which is why The OrbiTee shows this date. But at the moment shown on The OrbiTee, it is no longer December 21 in Moscow.
At or around this same date, Quito, Cape Town, and the remainder of the Southern Hemisphere experience their summer solstice, because this Hemisphere points towards the Sun.
To complete the picture, the two points in the Earth’s orbit where neither Hemisphere points towards or away from the Sun, but rather at a right angle, are the spring and autumn equinoxes, generally occurring in the Northern Hemisphere March 20 and September 22, respectively, and on the reverse dates in the Southern Hemisphere.
The OrbiTee shows the Earth on March 20, where it is normally the northern spring equinox at all cities listed.
And the OrbiTee shows the Earth on September 23, when the northern autumn equinox normally falls in Mumbai, Tokyo, Wellington, and Kiritimati. At the moment shown here, it is not yet September 23 in Honolulu, on the other side of the International Date Line.
There are many great explanations of how the tilt of the Earth causes the seasons, both in print and online; the latter including convincing animations from USA Today and Prentice Hall.
The OrbiTee is designed to drive home this lesson in a way you’ll never forget, placing you at the center. The Earth encircles the Sun lying inside your torso. It’s easy to see how, by maintaining the same tilt, the Earth hemispheres point towards or away from or at a right angle to the Sun on the four key dates of the year. The OrbiTee shows what portion of the Earth is lit up, and what lies in shadow. Great web sources for this info include Foumilab’s Earth and Moon Viewer and Paul Carlisle’s Earth Viewer. (We updated the imagery on these sites with data on current Arctic sea ice melt.) The area in the middle of the lit portion receives the most direct sunlight and so tends to be warmest, as confirmed by the time/temperature readings at five representative locations on each Earth. The Time and Date site provides times around the world relative to any other time, while Weather Underground’s site plots temperatures around the world going back some years.
The five locations on each Earth provide a good spread over the visible Earth; constitute major population centers (Mumbai, Shanghai, Tokyo, etc.), represent places of long-standing geographical interest (Greenwich, of course; Jerusalem, once said to occupy the center of the world; Quito, the major city on the Equator, etc.), or otherwise illustrate particularly well in the views shown the impact of the Earth’s tilt. For example, at 21:00 (9pm) on June 21 in Denver, it’s getting dark, but further east, in Repulse Bay on the Arctic Circle, it’s still light at 10pm (and will be so 24/7 until July 10).
Meanwhile, in the September view, Kiritimati is the easternmost inhabited point in its time zone—which, through its unilateral action—is just to the west of the International Date Line. As such, it’s also the easternmost inhabited place on Earth, with its people experiencing the new year earlier than anywhere else (at least while people remain, as rising sea levels stemming from global warming may soon render Kiribati—of which Kiritimati is the capital—uninhabitable.) Though slightly west, Honolulu lies to the east of the Date Line. Its time is 24 hours earlier than Kiritimati’s.
The Moon’s position relative to Earth varies from year to year, so The OrbiTee goes by 2009, when we would see it from our vantage point on March 20 and December 21 —but not on June 21 or September 23, when the Earth hides it. Its proportions of light and shadow on the former dates would seem to mirror Earth’s. The size of the Moon as portrayed by The OrbiTee is in scale with the size of the Earth. And the Moon’s orbit around the Sun, as also portrayed, does not involve orbiting the Earth. Rather, the Moon and Earth seemingly drive around a kind of race track, switching lanes and changing speeds in a way that makes it seem the Moon orbits the Earth. The Sun’s gravitational pull so exceeds that of Earth that the Moon continually moves forward around the star, not our planet.
The stars portrayed by The OrbiTee are those visible from our vantage point at the respective dates. May 6, 2009, in between March 20 and June 21, in the middle of the tee front (i.e., the stars that would be visible from the far side of the Earth could we block out the Sun, as in a solar eclipse); November 6, in between September 23 and December 21 in the middle of the back; February 6, in between December 21 and March 20, and so the sliver to the right of the December Earth and the sliver to the left of the March Earth; and August 8, in between June 21 and September 23, and so the sliver to the right of the June Earth and the sliver to the left of the September Earth. Starry Night software has been the standard popular source of star views, at least until Microsoft and Google provided the info free online.
The stars above and below the location listings are merely decorative.
The Sun would normally appear from the vantage point of The OrbiTee at least as large as the Moon. These look the same size viewed from the Earth, with dramatic differences in actual size counterbalancing dramatic differences in distance from the Earth. The Sun’s larger actual size would allow it to retain its scale more than the Moon as we pull pack from the Sun, Earth, and Moon, as in The OrbiTee vantage point. But portraying the Sun this large would not allow it to appear sufficiently far from the Earth (or Moon) on The OrbiTee. As in any portrayal of these three celestial bodies, drawing to scale is impossible.
Join beyondadornment on Facebook