Understanding the Timing of Sunrises and Sunsets Throughout the Year
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When discussing the longest day of the year, commonly recognized as the summer solstice, it’s essential to note that this day does not feature the earliest sunrise or the latest sunset. The amount of daylight we experience on Earth fluctuates significantly throughout the year, despite the consistent 24-hour day length. At the summer solstice, locations closer to the poles enjoy extended daylight, while near the equator, the duration of sunlight remains nearly uniform throughout the year. This raises a question from Patreon supporter Ben Turner:
> Though solstices mark the longest and shortest days, when do the earliest and latest sunrises and sunsets occur? Are these timings consistent across different latitudes?
The answer is complex and varies by location. Let’s delve deeper into this topic.
The phenomenon known as the analemma illustrates the apparent movement of the Sun throughout the year when viewed from a fixed point on Earth at the same time each day. The resulting figure, which appears as a figure-eight, varies depending on the time of day the observations are made. For instance, at noon, the analemma would appear vertically aligned, while earlier or later in the day, it tilts either counterclockwise or clockwise.
The summer solstice corresponds to one end of the analemma, while the winter solstice aligns with the opposite end.
The analemma's distinctive shape arises from two interconnected factors:
- The Earth's axial tilt of 23.5° relative to its orbit.
- The elliptical orbit of the Earth around the Sun.
If the Earth’s axis were perfectly vertical, and its orbit a perfect circle, the analemma would simply be a point, indicating the Sun's consistent path. However, the combination of the axial tilt and elliptical orbit leads to the variation we observe.
As the Earth follows its elliptical orbit, the speed of its revolution around the Sun fluctuates. Near aphelion (farthest from the Sun), the Earth moves more slowly, leading to a significant deviation in the timing of sunrises and sunsets. Conversely, near perihelion (closest to the Sun), the Earth moves faster, resulting in a more compressed timeline for these events.
Consequently, during the December solstice, the changes in sunrise and sunset times are more pronounced compared to the June solstice, due to these overlapping effects of axial tilt and orbital shape.
The equation of time accounts for variations in solar time based on the Earth’s motion and its axial tilt. As the year progresses, the timing of sunrises and sunsets shifts, particularly noticeable close to the solstices. In the northern hemisphere, the earliest sunrises occur several days prior to the solstice, varying by latitude, with similar patterns evident for sunset timings post-solstice.
The nature of these shifts also applies inversely in the southern hemisphere, where the latest sunrises align with the shortest days. Notably, the shifts are not symmetrical; the changes around the December solstice are generally larger than those during the June solstice.
Finally, the analemma is not a fixed feature. In thousands of years, its shape will transition from a figure-eight to a teardrop as the Earth’s perihelion and aphelion align with equinoxes. This future adjustment will result in the earliest sunrises and latest sunsets coinciding with the summer solstice, while the reverse will occur during winter solstice.
Understanding these intricate dynamics helps clarify how our day-to-day experiences with sunlight vary throughout the year.
Please send your questions for “Ask Ethan” to startswithabang at gmail dot com!
Starts With A Bang is featured on Forbes and Medium, supported by our Patreon community. Ethan has authored "Beyond The Galaxy" and "Treknology: The Science of Star Trek from Tricorders to Warp Drive."