5:23 p.m., Dec. 21, say hello to Old Man WInter

Brighter days ahead as Winter Solstice marks seasonal change

Thu, 12/20/2018 - 2:00pm

    It is time to celebrate the return of light to the 44th latitude, and say goodbye to the days when skies don't lighten until 7 a.m. and dusk descends at 3:30 p.m. At approximately 5:23 p.m. Friday, Dec. 21, the North Pole will be tilted at its most distant point all year from the sun, at approximately 23.4 degrees.

    Sunrise is to be at 7:11 a.m. and sunset, 4:02 p.m.

    It is a particularly good moment to celebrate, because what follows is the northern hemisphere's gradual tilt back toward the sun as earth spins on its axis around the sun. Our days will get incrementally longer and by summer solstice on June 21, the skies in Maine won't truly darken until 10 p.m., or so.

    Tomorrow does, however, mark the first day of winter, and there is a fierce rainstorm on the horizon.

    According to TimeandDate.com, winter solstices can happen on December 20, 21, 22 or 23, though December 20 or 23 solstices are rare. The last December 23 solstice was in 1903 and will not happen again until 2303.

    And according to EarthSky.org: "You might notice how low the sun appears in the sky at local noon. And be sure to look at your noontime shadow. Around the time of the December solstice, it's your longest noontime shadow of the year."

    From the U.S. Naval Observatory

    The Tilt of the Earth's Axis and Its Elliptical Orbit
    We have all been taught that the seasons are caused by the 23.4° angular offset between the earth's axis of rotation and the perpendicular to the earth's orbital plane with the sun (see obliquity below). The earth's rotational axis stays nearly fixed in space, even as the earth orbits the sun once each year.

    As a result, when the earth is at a certain place in its orbit, the northern hemisphere is tilted toward the Sun and experiences summer. Six months later, when the earth is on the opposite side of the sun, the northern hemisphere is tilted away from the sun and experiences winter. The seasons are, of course, reversed for the southern hemisphere.

    The solstices mark the two dates during the year on which the earth's position in its orbit is such that its axis of rotation is most tilted toward or away from the sun.

    These are the dates when the days are longest for the hemisphere tilted toward the sun (where it is summer) and shortest for the opposite hemisphere (where it is winter).

    However, there is a complication. The earth's orbit is very close to being a perfect circle, but not quite. It is somewhat elliptical, which means that the distance between the earth and the sun varies over the course of the year.

    This effect is too weak tocause the seasons, but it might have some influence over their severity.

    The earth reaches perihelion - the point in its orbit closest to the sun - in early January, only about two weeks after the December solstice. Thus winter begins in the northern hemisphere at about the time that the earth is nearest the sun. Is this important?

    Is there a reason why the times of solstice and perihelion are so close?

    It turns out that the proximity of the two dates is a coincidence of the particular century we live in.

    The date of perihelion does not remain fixed, but, over very long periods of time, slowly regresses (moves later) within the year. There is some evidence that this long-term change in the date of perihelion influences the earth's climate.

    The sun's track across the sky is much shorter and lower in the winter than in the summer. During the early fall, as the sun moves lower in the southern sky, sunrises get later, and sunsets earlier.