César E. Chávez Memorial Solar Calendar

Tribute to César Chávez & Dolores Huerta

Frequently Asked Questions

[0. Can I make a solar motion demonstrator?
See question 9 (the last question).]

  1. What is the official website of the Chavez Solar Calendar?  solarcalendar.org.
    ~
  2. Where does the Sun set on the equinoxes, solstices, and at various times of year? 
    See diagram below.
diagram showing position of sunset along the horizon as viewd from Chavez Sun Calendar site

3. What does “solstice” mean?
The word “solstice” comes from the Latin words “sol” (sun) and “stit” (stationary). Sunset and sunrise horizon positions do not change much from day to day around the solstice times. Likewise the elevation angle of the Sun at solar noon in the sky does not change much day to day around solstice times.  By contrast, equinoxes are the times of maximum rate of change of sunset and sunrise horizon positions as well as positions of the Sun at solar noon and number of hours of daylight.

4. When is the exact time of solstice?  
…for 2023 it’s Dec 21 at 7:27 pm Pacific time.

5. When is sunset on the winter solstice? 
In 2023 in Berkeley it’s Dec 21 at 4:53 pm.
Sunset is at about the same time one day later, Dec 22.

6. What celebrations happen around the times of solstices and equinoxes?

7. Since the winter solstice is the day with least hours of daylight and most hours of night, is the winter solstice the day of latest sunrise and earliest sunset?
No. The latest sunrise and earliest sunset of the year do not occur on the winter solstice.
Example—around the winter solstice of 2023 in Berkeley:

  • There are 9 hours, 32 minutes, and 20 seconds of sunlight on winter solstice (in Berkeley)
  • Latest sunrise(s) (7:25am) occur Jan 3-8 (6 days)
  • Earliest sunset(s) (4:49pm) occur Dec 2-10 (9 days)

WHY? There are two elements that contribute to unusual variations in timing of sunsets & sunrises:

  • tilt of Earth’s axis (23.5°with respect to its orbit) and
  • eccentricity of Earth’s orbit (elliptical shape deviates from a perfect circle by 3%)

Deviations in timings of sunsets are similar to deviations of apparent solar noon (the moment when the Sun crosses the north-south meridian line in the sky) from the mean solar noon (averaged over the whole year, as displayed in sundials). Deviations are described mathematically as the equation of time. Here is a graph showing the two components of the equation of time (tilt of Earth’s axis and eccentricity of Earth’s orbit):

Graph showing just the eccentricity and Earth's tilt components of the equation fo time.

…and here is a graph showing those two components added together to make the equation of time — the red line:

graph showing each of two components of the equation of time (tilt of Earth's axis and eccentiricity of Earth's orbit) and the sum of those two components displayed as a red line.

References:
https://in-the-sky.org/article.php?term=equation_of_time
https://en.wikipedia.org/wiki/Equation_of_time

8. What causes the seasons?  Let’s consider some common assumptions:

Assumption 1:  “seasons are caused by how close the Earth is to the Sun at different times of year.” 

In the diagram below, notice that

  • for the northern hemisphere Earth is closest to the Sun near the time of the winter solstice (December). The actual time of the closest separation of Earth and Sun is about January 3 each year, an event called perihelion
  • but for the southern hemisphere Earth is closest to the Sun near the time of the summer solstice (that also occurs in December).
Seasons diagram on gray background and explanatory text, showing relationship of Earth's tilt and the Sun at equinoxes and solstices.

See also: other variations of this seasons diagram (with less text) at the bottom of this page

So for people in the southern hemisphere, changes in the Sun-Earth distance may be a logical explanation for why it’s warmer in summer than in winter.  But the logic completely fails for those of us in the northern hemisphere.

That means our assumption that “seasons are caused by how close the Earth is to the Sun at different times of year” must be flat wrong! It’s not even the best reason for the southern hemisphere.

Assumption 2: One hemisphere nearer or farther from the Sun because of the tilt of the Earth.

The diameter of Earth is less than 15,000 km which is  negligible compared to the roughly 150 MILLION  km Earth-Sun distance. So assumption 2 is also flat wrong.

So what are the real reasons for the seasons? 

In the diagram above, Earth’s axis is always pointing towards the North Star, at a 23.5° angle from vertical to the plane of Earth’s orbit.  This results in two main effects that cause the seasons:

  • number of hours of daylight and
  • intensity sunlight related to the angle of the Sun’s rays striking the ground. The closer to vertical the Sun’s rays are relative to the ground, the more intense the light is.

Example: In the northern hemisphere’s winter, 

  • less of the northern hemisphere is illuminated by sunlight, causing fewer hours of daylight, more hours of night and
  • there is less intense sunlight due to lower angle of the Sun’s rays striking the ground.

9. Is there a nifty model that shows the Sun-Earth relationships that cause the seasons?

Try the Solar Motion Demonstrator 

The original Solar Motion Demonstrator was developed by Joe Snyder at Oberlin College and is available through the Astronomical Society of the Pacific store.

Northern Hemisphere Solstices and Equinoxes
(alternate diagrams)

Seasons diagram with black background and minimal text, showing relationship of Earth's tilt and the Sun at equinoxes and solstices.

The above diagram, with black background, is suitable for planetarium use.  Both that diagram and the one below do not include these important qualifications: (1) For southern hemisphere, reverse the solstice and equinox labels; (2) This is a side view diagram of Earth’s nearly circular orbit; (3) It’s not to scale—the Sun is really 100 times bigger than Earth in diameter.

seasons diagram, transparent background, showing relationahip of Earth's tilt with the Sun on the solstices and equnoxes