Understanding the Sun's Journey: Seasonal Variations and Sunset Directions

The movement of the sun across the sky is not only fascinating but also an essential part of our understanding of astronomy. The sun does not always set in the exact same spot, and this phenomenon is primarily due to the tilt of the Earth's axis and its orbit around the sun. Let's delve into the details of these seasonal variations and the degree of movement that the sun undergoes during different times of the year.

Seasonal Variation

During specific times of the year, the sun's position at sunset changes, leading to different directions where it sets throughout the year. These times are marked by the solstices and the equinoxes:

Summer Solstice

During the summer solstice, around June 21, the sun reaches its most northerly point in the sky, and it sets in the northwest in the Northern Hemisphere. This is the longest day of the year in the Northern Hemisphere and the shortest night.

Winter Solstice

Conversely, during the winter solstice, around December 21, the sun reaches its most southerly point and sets in the southeast. This is the shortest day and the longest night in the Northern Hemisphere.

Equinoxes

The sun sets almost due west during the equinoxes, which occur around March 21 and September 23. On these days, the day and night are of approximately equal length, and the sun shines directly over the equator.

Degree of Movement

The sun's position at sunset changes approximately 23.5 degrees north and south of true west throughout the year. This is a direct result of the Earth's axial tilt of about 23.5 degrees. Therefore, from the summer solstice to the winter solstice, the sunset point can shift by about 47 degrees across the horizon.

This variation is more pronounced at higher latitudes, while at the equator, the sun sets more consistently in the west throughout the year, only deviating by 23.4 degrees north and south of true west at a solstice. At the highest latitudes, such as 88 degrees north or south, the sun can set almost anywhere around you, but not exactly east or west.

Calculating Sunset Directions

The movement of the sun can be calculated using a simple formula that takes into account the sun's declination and the observer's latitude. Ignoring other factors such as atmospheric refraction, the size of the sun, and parallax, the formula for the sun's azimuth at sunrise and sunset is:

cos(azimuth) (sin(solar declination) ± sin(earth's axial tilt)) / cos(latitude)

The azimuth represents the direction of the sun. During solstices, the solar declination is at its maximum value, ±23.44 degrees. This formula helps us calculate the direction of the sunrise or sunset and shows how it changes with different latitudes and seasons.

The SunCalc website is a great resource that provides the direction to the sun during the day, the change in sunrise and sunset over time, and how these vary across different locations around the world. By dragging the time dot along the time bar, you can see how the sun rises and sets change over the course of a day. Changing the date allows you to see how these changes occur throughout the year.

Understanding the Sun's journey is not just a matter of scientific curiosity; it also has practical applications in navigation, agriculture, and even energy generation. By knowing when the sun will rise and set, people can make informed decisions about their daily activities and plan for the best use of solar energy. Whether you're an astronomer, a solar panel installer, or simply someone interested in the natural movements of the sun, this knowledge is invaluable.