Backyard Astronomy: The Basics of Our Sun and Moon

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. 

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

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.

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).

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.

At the Tropic of Cancer, the Sun peaks at the zenith at noon on the summer solstice.
The blue arc represents the Sun's path on the summer solstice.
The dark blue arc represents the Sun's path on either equinox.
The black arc represents the Sun's path on the winter solstice.

What is a Planet?

Why the Debate Over Pluto isn't Over

Portrait of Pluto captured by NASA's New Horizons spacecraft.
Image Credit: NASA/New Horizons
(Click to Expand)

I named this blog 'Earth is a Planet' because I believe it is important to maintain perspective about our place in the universe. As I discussed in my previous post, Earth is not the world, located at the center of the universe, but one of many worlds around one of many billions of stars within one of many billions of galaxies. In the vastness of space, the view that our human experience applies to the cosmos at large is simply misguided. It is for this reason that we as Earthlings must look to other planets for knowledge as opposed to vice versa. But what exactly is a planet? While the question may seem simple at face value, the definition of a planet is historically fuzzy and has evolved with our understanding of our Solar System and with our discovery of worlds around other stars. Perhaps the most widely known debate over this very topic in modern times relates to the classification of Pluto after the International Astronomical Union (IAU) updated the scientific community's formal definition of a planet back in 2006. The major consequence of the IAU's resolution, which disappointed much of the public, was the reclassification of Pluto from 'planet' to 'dwarf planet.'  We will return to the present definition of a planet later in this post, but it is important that we first discuss the historical context.

The word planet is derived from the ancient Greek word for 'wanderer,' and originally referred to the five 'stars' that appeared to meander across the night sky, separately from the others. These planets were eventually named Mercury, Venus, Mars, Jupiter, and Saturn (Uranus and Neptune, being further away, are not visible with the naked eye and were not discovered until 1781 and 1846, respectively). At this point in history, the Earth was assumed to define the center of the universe, and early astrologers included the Sun and Moon among the original set of planets (Fun fact: the days of the week are named after the seven original planets). Eventually, Copernicus' findings revealed that the Earth (along with Mercury, Venus, Mars, Jupiter, and Saturn) was a planet that orbited the Sun, and that the Moon orbited Earth. Ultimately, this heliocentric model of a Solar System (named after our Sun, a.k.a. Sol) replaced the original geocentric perspective, and a planet was redefined as anything orbiting the Sun.

Geocentric arrangement of the seven classical planets. Sizes not to scale. Obviously.
(Click to Expand)

By the time Pluto was discovered in 1930, our model for the Solar System incorporated the Sun and nine planets. Pluto maintained planetary status until it was later reclassified as a 'dwarf planet' by the IAU in 2006. To this day, many within the wider public remain disappointed and confused by the 'death' of our ninth planet.  To fully understand the decision, it is helpful to discuss another former planet with a similar history between Mars and Jupiter. Discovered in 1801, Ceres was labeled as a planet for half a century. Within the following ten years, three similar (former) planets were discovered within the same neighborhood of the Solar System - Pallas in 1802, Juno in 1804, and Vesta in 1807. As the region between Mars and Jupiter continued to fill with newly discovered objects, it became increasingly apparent that labeling them all planets would be unwise given how similar they all were to each other. By the mid-1800s, Ceres, Pallas, Juno, and Vesta were reclassified as asteroids within the asteroid belt, and their fifteen minutes of fame in the classroom were over.

Simplified representation of the Solar System from 1930-2006. Not to scale.
(Click to Expand)

By the end of the 20th century and into the early 21st, Pluto's status as a planet grew fragile with the observation of a number of similar icy objects past the orbit of Neptune. Nevertheless, Pluto remained the largest of these Trans-Neptunian Objects; until another, similarly-sized world could be found in its neighborhood, there was no reason to reclassify it. With the discovery of Eris in 2005, planetary astronomers were having Ceres flashbacks that could no longer be ignored, and a convention was arranged to clarify the definition of a planet. Under the 2006 resolution, a planet is defined as a celestial body that is:

1. in orbit around the Sun,¹
2. massive enough to be spherical (as opposed to a shape more like that of a lumpy potato, like most asteroids), and
3. gravitationally dominant within its orbit, such that it has cleared the neighborhood.

Pluto, Eris, and Ceres, having not met the third criterion, were thus classified as dwarf planets.² Similarly to the gradual discovery of the asteroid belt, the observation of additional icy objects within the same region of our Solar System led to the recognition that Pluto was one of many objects within the Kuiper Belt.

Simplified representation of the Solar System after 2006. Not to scale.
(Click to Expand)

Setting aside the classification intricacies, however, it is clear that the definition of a planet is far from fixed, and it will likely have to be refined with future discoveries in our own Solar System and beyond. The rapidly growing catalog of extrasolar planets around other stars (a.k.a. exoplanets) has made it even more difficult to classify the diversity of observed worlds. Planetary systems are far more complex than anyone could have predicted even just a century ago, and there are not always distinct categories that individual objects can be sorted into. Perhaps there will be a day when a formal definition for the term 'planet' becomes cumbersome and unnecessary. Regardless of how Pluto is ultimately classified, New Horizons has shown us that it is a pretty awesome world in our Solar System that is just as worthy of admiration and exploration as any of the eight major planets.

Fun Links:
- Scale model of the Solar System
- All about the seven new planets discovered around TRAPPIST-1

¹ Does not apply to extrasolar planets

² Pallas, Juno, and Vesta are not dwarf planets because they do not meet the second criterion. Satellites like our Moon are not considered dwarf planets.