Solar Calendar

Frequently Asked Questions about…

Berkeley’s Solar Calendar—Tribute to César Chávez & Dolores Huerta  (solarcalendar.org):

1. What do “solstice” and “equinox” mean?
2. Where is the sunset on the equinoxes, solstices, and at various times of year?  (Solar Motion Demonstrator)
3. When is the exact time of solstice/equinox? 
4. When is sunset on the solstice/equinox? 
5. What celebrations happen around the times of solstices and equinoxes?
6. Is the winter solstice the day of latest sunrise and earliest sunset?
7. What causes the seasons? 
8. What is a Shepherd’s Dial?

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1. What do “solstice” and “equinox” mean?
In simplest terms, the two equinoxes (spring/vernal and fall/autumnal) are the times when the number of hours of daylight is equal to the number of hours of night. The winter solstice happens when the number of hours of daylight is a minimum for the year; the sunrise and sunset points are farthest south (in northenr hemisphere); and the Sun a noontime is lowest in the sky for the year. The summer solstice happens when the number of hours of daylight is maximum; the sunrise and sunset points are farthest north (in northern hemisphere); and the Sun a noontime is highest in the sky for the year. See Solar Motion Demonstrator.
The word “solstice” comes from the Latin words “sol” (sun) and “stit” (stationary). The elevation angle of the Sun at solar noon as well as the sunset and sunrise horizon positions do not change much from day to day around the 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.
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2. Where does the Sun set on the equinoxes, solstices, and at various times of year? 
Try making and playing with a Solar Motion Demonstrator, a model of Sun-Earth relationships:

• 12 page version (with questions, for classrooms)
• 1 page version (larger size)
• Jumbo demo size (assemble from 3 pages)

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

Here are the sunset positions as seem from the Berkeley Chavez/Huerta Solar Calendar:

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3. When is the exact time of solstice/equinox?  
2024:
Winter Solstice is December 21 at 1:19 am Pacific time.
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4. When is sunset on the solstice/equinox? 
In 2024 in Berkeley winter solstice Dec 21 sunrise is 7:21 am and sunset is at 4:54 pm.
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5. What celebrations happen around the times of solstices and equinoxes?

• Winter solstice – See PDF.
• Vernal equinox – See PDF. See also Youtube video about Nowruz
• Summer solstice – Articles: Euro News; Xiazhi – China; Pagan Europe; Sweden; Juhannus – Finland;  
• Autumnal equinox – See timeanddate article.
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6. 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 2024 in Berkeley:

• There are 9 hours, 32 minutes, and 20 seconds of sunlight on winter solstice
• Earliest sunset(s): 4:49 pm, Dec 2-10 (9 days)
• Latest sunrise(s): 7:25 am, 2025 Jan 3-7 (5 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%)
• the relationship between the two factors is complicated because the direction of Earth’s axis tilt in space is not in the same direction as the closest approach of Earth to the Sun (called perihelion) that occurs on or around January 4 each year, about two weeks AFTER the solstice. The speed of Earth in its orbit varies and is maximum around the time of perihelion. This affects timing of sunrises and sunsets as well as the time of solar noon (the moment when the Sun crosses the north-south meridian line in the sky, when it is highest in the sky on a given day).

Deviations in timings of sunsets are similar to deviations of apparent solar noon from 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):

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

References:
https://in-the-sky.org/article.php?term=equation_of_time
https://en.wikipedia.org/wiki/Equation_of_time
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7. 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).

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 dead wrong! It’s not even the best reason for the southern hemisphere.

Assumption 2: One hemisphere is nearer or farther from the Sun because of the tilt of the Earth. The part that’s tilted toward the Sun is closer to the Sun.

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 dead 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.
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8. What is a Shepherd’s Dial?

It’s a cylindrical sundial. Make your own Shepherd’s Sundial.

Northern Hemisphere Solstices and Equinoxes — alternate diagrams

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 over 100 times bigger than Earth in diameter; and the distances are hopelessly too small to reflect the true scale of the orbit.