As we go about our daily lives, it is easy to take our Sun and Moon for granted. After all, the Sun rises every morning and sets every evening, while the Moon cycles through its familiar phases each night, starting over about once per month. Although these natural rhythms are ingrained in our lives and in our biology here on Earth, many overlook the beauty in the details that can reveal so much more about our planetary framework. By looking up at the sky with curiosity from time to time, it is possible to visualize the arrangement of the Earth, Moon, and Sun throughout the daily, monthly, and yearly cycles that define our calendar system.
Daily Cycles
The Sun's Path in the Sky
Monday, March 20 was the Vernal (Spring) Equinox in the Northern Hemisphere (which means that it was the Autumnal Equinox in the Southern Hemisphere). I will elaborate on what this means when I discuss annual cycles below, but for this section, what is important to emphasize is that day and night are of equal length on the equinoxes. This means that for all locations on Earth during the equinox, the Sun will rise directly east before setting directly west 12 hours later. The path that the Sun follows over the course of the day, however, differs based on latitude. In the Northern Hemisphere, its arc is tilted southward, whereas in the Southern Hemisphere, the opposite is true. At the equator, the Sun passes directly overhead along a straight path, and at the poles, the Sun circles along the horizon. The angle of the Sun's arc with respect to vertical is equal to the latitude of the observer.
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| Depiction of the Sun's path across the sky during equinox from the Northern Hemisphere (Blue), the equator (Red), and the Southern Hemisphere (Green). |
During the summer and winter months in each hemisphere, the length of the day varies as its path shifts. I will discuss this further when we get to annual cycles.
Challenge: Determine your latitude by measuring the angle at which the Sun peaks in the sky. Is your measurement accurate? Why or why not?
The Moon Looks Flipped in Opposite Hemispheres
Although the Moon's orbit is inclined with respect to the Earth's equator, the same principle applies. Because the angle of its path is flipped across the equator, observers from one hemisphere will notice that it appears upside down in the other.
Monthly Cycles
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| Arrangement of the Sun, Earth and Moon during each of the lunar phases. In this diagram, the Moon orbits and the Earth spins in the counterclockwise direction. (Click to Expand) |
Lunar Phases
The lunar phases have a cycle that lasts approximately 29.5 days, or one month, and relates to the Moon's position with respect to the Earth and Sun:
- When the Moon is between the Earth and Sun, its phase is referred to as a "new moon." During this phase, the Moon is near the Sun in the sky, and is not visible at night.
- During the waxing crescent phase, the Moon is further from the Sun, and is visible as a slender crescent during the few hours after sundown before setting, itself.
- At first quarter, the Moon is half illuminated and is visible until it sets at midnight.
- During the waxing gibbous phase, most of the Moon is now visible before it sets during the early morning hours before dawn.
- The full moon rises near sundown and sets near sunrise.
- During the waning gibbous phase, the Moon rises between sundown and midnight, and sets between sunrise and noon.
- At last quarter, the Moon rises at midnight and sets at noon.
- The waning crescent phase is only visible in the hours before sunrise.
Why Don't We See Eclipses Every Month?
If the Moon were to orbit Earth along the same plane as the planet around the Sun, each new moon would coincide with a solar eclipse, and each full moon would obscured during a lunar eclipse. Those who keep up with these events know that this is not the case. The fact that eclipses are substantially less frequent is due to the fact that the Moon's orbit is inclined with respect to the planet's orbit.
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| Earth's axial tilt and the Moon's inclination. Sizes/distances not to scale. (Click to Expand) |
Annual Cycles
Axial Tilt and Seasons
In addition to the Moon's orbit, Earth's spin axis is tilted by 23.4°, thereby creating seasonal cycles as the planet orbits the Sun. At the June solstice, the North Pole is angled toward the Sun in permanent daylight, whereas the South Pole is angled away from the Sun in darkness. The reverse is true during the December solstice. The Arctic and Antarctic circles (which are located at 90° - 23.4° = 66.6°N/S) represent the minimum latitudes that experience permanent day or night at solstice. On the summer solstice, the Sun peaks directly overhead at the zenith along the Tropic of Cancer (23.4°N) or the Tropic of Capricorn (23.4°S), where the rays of sunlight are directed perpendicularly to the ground at noon. The shift in climate that defines our seasons are a result of longer days during the summer months, and longer nights during the winter. Between the two solstices are the equinoxes, when day and night are both 12 hours long at all latitudes (except the poles, where the Sun follows the horizon).
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| Depiction of Earth at solstice and at equinox. (Click to Expand) |
The Sun's Path in the Sky
As the length of the day changes with the seasons, so does the Sun's path in the sky. While the angle of the arc remains constant for a given latitude, the plane that the Sun traces shifts northward or southward over the course of the year, thereby increasing or decreasing the hours of daylight. Additionally, the Sun will peak higher in the sky during the summer months, allowing its rays to reach the planet's surface with greater intensity. Likewise, the Sun will be lower in the sky over the winter, heating the surface less intensely and over the course of fewer hours.
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