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The Earth’s seasons

The Earth’s seasons are caused by three factors:

  • The Earth orbits the Sun once a year in a nearly circular orbit.
  • The Earth’s axis of rotation (the straight line through the center of the Earth between the north and south poles) is not perpendicular to the plane of the Earth’s orbit. The Earth’s axis is tilted by about 23.4° from the the direction perpendiular to the orbital plane.
  • The orientation of the Earth’s axis in space remains nearly constant even as the Earth revolves around the Sun. It always points in the general direction of the star Polaris.
Sun track

sun track

The result is that when the Earth is on one side of its orbit, the south pole is tilted toward the Sun (by as much as 23.4°) and the southern hemisphere experiences summer. Six months later, when the Earth is on the opposite side of its orbit, the north pole is tilted toward the Sun (by as much as 23.4°) and the northern hemisphere experiences summer. (Views of the Sun’s illumination on the Earth on any date are available here.) What we see from our viewpoint in the Earth’s northern hemisphere is that the Sun’s apparent daily track across the sky is much higher (that is, more northerly) in summer, and lower (more southerly) in winter. From horizon to horizon, the Sun’s track is longer in summer and shorter in winter; so that in summer, sunrises are much earlier and sunsets are much later than in winter. See, for example, the graphic above, or this photograph of the Sun’s paths through the sky at different times of the year.

The great circle The great circle

So we are used to the fact that the length of daylight is significantly longer in summer than winter, and most of us know that the “longest day” (that is, the day when the Sun is above the horizon the longest) is the summer solstice, around June 21, when the Sun has reached its most northerly and longest track in our sky; and the “shortest day” is the winter solstice, around December 21, when the Sun has reached its most southerly and shortest track in our sky.

It would make sense, then, for the summer solstice to also be the date at which sunrise is earliest and sunset is latest; and for the winter solstice to be the date when sunrise is latest and sunset is earliest. However, that is not what happens! Nature sometimes defies our expectations.

The local meridian is a great circle passing through the celestial poles and through the zenith of an observer’s location on the planet. Image Credit: Daniel V. Schroeder

And that is because we have not talked about one other factor in sunrise and sunset times that is not at all obvious. It is that the Sun moves across the sky, in its apparent daily track, at slightly different rates at different parts of the year. Most of the Sun’s east-to-west apparent motion in the sky is caused, of course, by the rotation of the Earth, which is quite uniform (to milliseconds per day). But a small part of the Sun’s apparent daily motion depends on the position of the Earth in its orbit around the Sun. This component of the Sun’s apparent motion varies by a small amount over the course of a year due to the elliptical shape of the Earth’s orbit and to the tilt of the Earth’s axis.

Earth’s axial tilt

Earth’s axial tilt

Continue reading The Earth’s seasons

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Super Blue Blood Moon

Super Blood Moon 2018
Super Blood Moon 2018
Super Blood Moon 2018

The rare Super Blue Blood Moon is about to happen for the first time in 150 years!

Early Wednesday morning brought a lunar event that hasn’t been seen since 1866.

It was at least partially visible in all 50 U.S. states, though the views were better the farther west you live.

Let’s break this down. This event – called a super blue blood moon – was actually three fairly common lunar happenings all happening at the same time.

And scientists say that information gathered during the event could help them figure out where to land a rover on the moon.

Continue reading Super Blue Blood Moon

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Photomosaic of Triton

Photomosaic of Triton
Photomosaic of Triton – Click to Enlarge
Photomosaic of Triton
Unidentified photographer
Medium unknown
Date unknown
National Air and Space Museum, Center for Earth and Planetary Studies
Accession no. P34754This photomosaic of Triton, Neptune’s largest moon, was assembled from fourteen individual frames to show the great variety of its surface features. At the bottom of the image are remnants of the south polar cap, which contain dark streaks generally aligned toward the northeast. Even though these features are darker than others on Triton, they reflect nearly ten times as much light as the surface of the Earth’s moon. The region that scientists have informally dubbed the “cantaloupe” terrain, as a result of its small dimples with upraised rims and shallow central depressions, is visible north of the cap, in the western half of the disk. Long fractures that have opened and allowed some icy material to ooze up and form a central ridge crisscross the region and extend into parts of the polar cap region. Toward the south this terrain is covered with a light layer of frost. 

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Pond with Three Cows and a Crescent Moon – (Jean-Baptiste-Camille Corot – circa 1850)