Hubble's Universe Unfiltered

  • January 5, 2017

    Happy Orbital New Year!

    by Frank Summers

    It is January and the start of a new year - at least according to the Gregorian calendar. There are many other "new year" celebrations one can have on many other dates. Most people have heard of the Chinese New Year (January 28 this year) or the Jewish New Year of Rosh Hashanah (September 21-22 this year). There was even that business about the Mayan calendar doing a total reset back in 2012 - sort of a super duper new year's event - with its completely failed predictions of dire consequences. Lots of other New Year celebrations are listed on this Wikipedia page.

    With so many to choose from, one can see that the start of a new year is just an arbitrary selection of a point in time. It is really no different from any other point in time, except that we give it meaning. For many, including me, the innate recognition of this fact has always made the celebrations a little hollow. The real meaning is found in looking back at past events and accomplishments with an eye toward continuing and improving in the future. It doesn't really matter when such reflection and planning is done, though it is nice to have a period where it becomes a topic of general conversation.

    In science, we think of the selection of an arbitrary measure as establishing the zero point of a scale. For example, it is well known that zero degrees on the Celsius temperature scale is equivalent to 32 degrees on the Fahrenheit scale. These two temperature scales have different zero points, and indeed, different sizes for their measures of a degree. There are many situations involving continuous measures (such as distance, energy, and pressure) where a zero point needs to be established, and all other measures are relative to that point.

    Temperature, like time, is a continuous measure. However, the fixed points of temperature have a physically motivated basis. The freezing and boiling points of water are used to establish the Celsius and Fahrenheit scales. If applied to the new year, what physical basis might one choose?

    The Chinese New Year is also called the Lunar New Year, because it is based on the motion of the Moon. The new year is marked by the Moon passing through the new moon phase on the first lunar month of the calendar. That date will vary between late January and late February, because the Moon's orbital period and Earth's orbital period are not integer multiples. The synodic period of the Moon is about 29.5 days long, making for about 12.4 lunar obits every year.

    Considering that a year is the completion of an Earth orbit, an Earth-based physical basis seems more proper. The orbital points in Earth's motion that most people have heard about are the solstices and the equinoxes. These points occur when Earth's rotational axis points toward/away from the Sun (solstices) or perpendicular to the Sun (equinoxes).

    The new year occurs less than two weeks after the December solstice (generally on the 21st or 22nd). In the northern hemisphere, winter solstice marks the shortest daylight of the year, and it seems fitting to measure a year as growing brighter from that low point. Of course, in the southern hemisphere, that same date marks midsummer's night, and the time of the longest daylight of the year. If the Gregorian calendar had been established in Australia, it might be shifted by six months.

    Astronomers do use the vernal equinox as a zero point, but for spatial coordinates, not time. The rotational axis of Earth precesses on a 26 thousand-year cycle. Hence the position of the north pole on the sky (the north celestial pole) varies over time. Right now it is pointed very close to Polaris, the north star. In about 12 thousand years, Polaris will be well away from the pole and Vega will be the new north star. The zero point of latitude on the sky, called declination, is set by these slowly shifting poles. The zero point of longitude on the sky, called right ascension, is not as easily defined. Astronomers use the position of the Sun at the vernal equinox. That point shifts over time as the rotational axis shifts (called the precession of the equinoxes) and results in some interesting conventions for specifying celestial coordinates, which need be left to another blog post.

    Overall, using either solstice or equinox to define a new year seems a bit askew. Instead of using Earth's rotational axis, wouldn't it be more appropriate to use characteristics of Earth's orbit?

    Earth's orbit is very nearly circular, but is technically an ellipse. The two significant orbital points are the closest approach to the Sun, called perihelion, and the most distant point, called aphelion. The difference in distance from the Sun at these points is relatively small, about 1.7 percent of Earth's average distance. Still, on the scale of the solar system, that's a distance of about 2.5 million kilometers (1.5 million miles).

    Perihelion is, to me, the natural point in the orbit to use as the start of a new year. It is the beginning of Earth's next passage around the Sun. This year, 2017, perihelion occurs on January 4 at 2:17 PM UTC. We just passed through it as I was writing this article. Happy Orbital New Year!

    Those in the northern hemisphere may find it strange that Earth is closest to the Sun during winter. However, simply remember that it is summer at this same time in the southern hemisphere, and it becomes apparent that orbital distance is not relevant to the seasons. Seasons are only marginally affected by Earth's orbit, but instead are controlled almost exclusively by Earth's axial tilt. The seasonal cycle derives from solstice and equinox, not perihelion and aphelion.

    While I do like perhelion as the start of a new year, I cannot advocate basing a calendar on it. The reason is simply that our lives are more governed by the length of a day than that of a year. These time periods are, again, not integer multiples, with about 365.2422 days in a year. Hence, the date of perihelion is not constant in the Gregorian calendar. Also, Earth's orbit is precessing such that the perihelion point moves relative to distant stars on a timescale of over 100 thousand years. Combined with the rotational axis precession, the date of perihelion progresses through the seasons about every 23 thousand years. Our lives are much more affected by the cycle of the seasons than they are by Earth oscillating between perihelion and aphelion.

    One is left with the conclusion that a date for the start of a new year based on Earth's orbit s a nice idea, but impractical. An arbitrary date is actually the best choice, and can be adjusted as needed to keep in line with the changing celestial motions (e.g., the addition of leap-years and even leap-seconds). Our current choice has a solstice just before and a perihelion passage just after. That suitable confluence gives us an extended period over which to celebrate yuletide. I hope yours was memorable, and wish you the best during our next planetary orbit.