Chavez Sun Calendar

The official website of the Chavez Solar Calendar is solarcalendar.org.

The word "solstice" comes from "sol" (Latin fors sun) and "stit" (Latin for "stationary"). This alludes to the fact that sunset and sunrise positions do not change much around the solstice times. Likewise the position of the Sun at solar noon in the sky does not change much around solstice times. By contrast, equinoxes are the times of maximum rate of change of sunset positions, sunrise positions, positions of the Sun at solar noon, and number of hours of daylight.

Winter solstice gathering materials

Celebrations from many cultures—winter solstice
Why do the latest sunrises and earliest sunsets of the year not occur on the winer solstice?
For winter of 2019:
- Latest sunrise(s) (7:25am) occur Jan 3-8 (6 days)
- Earliest sunset(s) (4:49pm) occur Dec 4-10 (7 days)
- Sunset on Dec 21 is 4:53pm and is only the same on one adjacent day, Dec 20.
- 9:32:21 hours of sunlight on winter solstice
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 (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 referred to as the equation of time. Graphs of the equation of time: 2 components --|-- components added
[references: https://in-the-sky.org/article.php?term=equation_of_time and https://en.wikipedia.org/wiki/Equation_of_time]

Summer solstice gathering materials

Celebrations from many cultures—summer solstice

Solar Motion Demonstrator — 1 page version --|-- 12 page version
Sunset positions throughout the year

The Reason for Seasons

Many people assume that the seasons are caused by how close the Earth is to the Sun at different times of year.

As the diagram below shows, the Earth in its orbit is closest to the Sun near the time of the winter solstice (in northern hemisphere). The actual time of this closest approach of Earth to the Sun is about January 3 each year and that event is called perihelion. So for people in the southern hemisphere, when it's summertime in December and January, distance from Earth is a logical explanation for why it's warmer in summer than in winter. Though the change in distance seems like a lot (about 6 million km), it is only about 4% of the total distance (about 150 million km). The logic completely fails when you take into account that when it's summer in the northern hemisphere, it's winter in the southern hemisphere.

The real cause of the seasons is the tilt of the Earth's axis with respect to its orbit, as shown in the diagram. Earth's axis is always pointing towards the North Star, at a 23.5° angle from vertical to the plane of Earth's orbit. In the northern hemisphere's summer, more of the northern hemisphere is illuminated by sunlight, causing