Imagine the Universe!
Imagine Home  |   Ask an Astrophysicist  |  
Ask an Astrophysicist

The Question

(Submitted January 16, 1998)

I have a question regarding sunrise and sunset. I realize that the winter solstice on 21Dec is the shortest day of the year. Since this date, the days have been gradually getting longer. Sunset has been getting gradually later as expected, however, sunrise continued to come later until the first week in Jan. My question is: what is the cause of this asymmetrical distribution of daylight between sunrise and sunset?

The Answer

This is due to a phenomenon called "the equation of time".

Solar day is the length of time between one local noon (when the Sun is highest in the sky) to the next. As it turns out, the length of the solar day is not always 24 hrs (its average over the course of a year defines 24 hrs). The solar day would always be 24 hrs if the Sun 'moves' east against the background of fixed stars at a constant rate (for convenience, astronomers have invented 'Mean Sun' to do exactly that). The real Sun moves at a variable rate, however,

  • Because of the tilt of the Earth rotation axis relative to its orbit around the Sun (the obliquity), the same reason as for the changing length of daytime hours.
  • Because the Earth's orbit is elliptical and so it moves faster at perihelion (around Jan 2) than at aphelion (Jul 3).

    Both effects combine to create an offset in the time of local noon (and those of sunrise and sunset) by as much as +/- 16 min: this is the equation of time. Around winter solstice, the daily change in the equation of time happens to be more important than the daily change in the length of the day, causing the phenomenon you so keenly observed.

    The equation of time is often represented by a figure 8. That figure is called an 'analemma'. There is an actual photograph of an analemma at https://pantherfile.uwm.edu/kahl/www/Images/Weather/Other/analemma.html, which was taken by Dennis di Cicco.

    I hope this helps,

    Koji Mukai, David Palmer, and Tim Kallman
    For the Ask an Astrophysicist Team

    Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See http://imagine.gsfc.nasa.gov/docs/ask_astro/ask_an_astronomer.html for help on other astronomy Q&A services.

    Previous question
    Prev
    Main topic
    Main
    Next question
    Next
  • If words seem to be missing from the articles, please read this.

    Imagine the Universe! is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

    The Imagine Team
    Project Leader: Dr. Barbara Mattson
    Curator: Meredith Gibb
    Responsible NASA Official: Phil Newman
    All material on this site has been created and updated between 1997-2014.
    This page last updated: Wednesday, 17-Feb-2010 11:41:38 EST