Tag Archives: Solstice

An Interesting Astronomical Observation Project

Most people intuitively know that days are shorter and nights are longer during the winter months, and days are longer and nights are shorter during the summer months. For those people who don’t know why, but would like to develop an understanding of why that is, here is an interesting observation project that begins to tease out the answer.

As I write this on December 23, 2019, we are just two days past the winter solstice, or the astronomical first day of winter. This event coincides with the shortest day and longest night of the year. And, at around this time, the sun rises and sets at its southernmost point on the horizon. This is a great time to begin the project, and observe the rise and set points move northward over the next six months.

This project can work with observing just sunrises, just sunsets, or both. After deciding when events you can observe, the first thing to do is select one or two observation points that are readily and repeatedly available. One should be toward the east in the mornings for sunrises, and the other toward the west in the afternoons for sunsets.

For the most casual observer, make a mental note of the sunrise or sunset point of the horizon, and the time. This first observation is your baseline. Make this observation every couple of days, and compare them with your baseline observation. During the period between the winter solstice (around December 21st) to the summer solstice (around June 21st), an observer should note that the rise or set point moves northward as the winter and spring progress. The sunrise time should be earlier and the sunset time should be later during this progression. By the time you get to June, you’ll be surprised at how much the rise or set points have moved, and how much the time has changed as well. From June 21st back to December 21st, the rise or set point should be moving southward on the horizon while the rise time gets later and the set time gets earlier.

More sophisticated observers can use a compass to record the azimuth of the sunrise or sunset, and record their observations in a spreadsheet for later analysis. A magnetic compass that can read degrees or a smartphone app can do the job. If you’ve never used a compass, they are not difficult to learn, and there are many online resources. Also, be sure to record the time in Universal Coordinated Time (UTC) to eliminate any confusion that daylight savings time might impose.

As an example for us in the eastern time zone, to convert standard time (EST) to UTC, add five hours. To convert daylight savings time (EDT), add four hours. Be aware that from early May until mid-August, sunset times of 8 pm EDT (and later) will use the next days date. For instance, sunset at 8:36 pm on June 21st might be recorded as 2020-06-21 20:36 EDT. Converting this to UTC by adding four hours pushes the time past midnight, and results in 2020-06-22 00:36 UTC.

If anyone actually does the observations, and would like an interpretation of the results, I would be glad to work with you, or write more on this topic, just let me know.

Sagittarius (The archer)

Constellation SagittariusSagittarius, the archer, is a zodiacal constellation that is rather easily found because of its distinctive teapot asterism. It is one of the 48 constellations cataloged by the 2nd century astronomer Ptolemy. It is located on the ecliptic between Scorpius and Capricornus. It can also be found by starting at Altair (a Summer Triangle star) and tracing southward along Aquila’s long axis. As is situated on southern most point of the ecliptic, this constellation hangs low in the southern sky, reaching its highest nightfall ascension in August. The Sun’s arrival at the southernmost point of the ecliptic around December 21st marks Winter Solstice and the first day of Winter.

This constellation has the distinction of presenting the foreground stars in the direction of the dense center of the Milky Way galaxy, which is rich in Messier objects. As such, it is worth taking the time to scan this constellation with binoculars or a small telescope. Several well-known nebula can be found in Sagittarius to include the Lagoon Nebula (M8), the Horseshoe Nebula, the Omega Nebula (M17), the Trifid Nebula (M20), and the Small Sagittarius Star Cloud (M24). Other Messier objects include M18, M22, M23, M25, M28, M54, M55, M69, M70 and M75.

IAU Sagittarius chart, Sky & Telescope magazine, June 5, 2011.
IAU Sagittarius chart, Sky & Telescope magazine (Roger Sinnott and Rick Fienberg), June 5, 2011.

© James R. Johnson, 2014.

2014 Summer Solstice

The Summer Solstice occurs this year on June 21st at 6:51am ET. Although it occurs at a specific instant in time, there is nothing that is easily observed directly, but there are some indirect observables worth noting. I will use this discussion as an opportunity to explain the four seasons’ relationship to the ecliptic.

The neatest star chart that I could find is a .jpg of one that was flown on the Apollo 11 mission. Please use it as a reference for the discussion that follows.  http://www.hq.nasa.gov/alsj/a11/A11StarChart-S1.jpg

Before proceeding, let’s locate a few key points on the chart. First, locate the equator, which is the 0° straight horizontal line across the center of the page. Note that at the left and right ends of this line are labeled “Vernal Equinox.” The Vernal Equinox, or Spring Equinox, is actually only one point, but since the equator is a circle that closes on itself, it is displayed here as two points. Next locate the ecliptic. This the sine wave that begins at Vernal Equinox at the left end of the equator and rises above it to about 23° before sloping back down. This highest point on the sine curve is the Summer Solstice, which is not labled on the chart. Proceeding toward the left past the Summer Solstice the ecliptic curves down toward and crosses the equator at mid-page. This point is called the Autumnal Equinox. Continue following the ecliptic to the left until it reaches it’s lowest point, which is called the Winter Solstice. Beyond this point, the ecliptic slopes back toward the north, reaching the equator again at the Vernal Equinox, thus completing the circle. Now go back to the beginning of the ecliptic at the left side of the page. Follow it to the left again, this time noting how many of the Zodiacal constellations you can find. I see nine of them, so three of the twelve constellations are not represented.

I have mentioned the ecliptic as the imaginary line among the background stars that marks the Sun’s path among them. In other words, if the Sun were just an ordinary bright star, we could see it and the background stars at the same time. If one plotted the Sun’s daily position on a star chart for a year and connected the dots, this line would represent the ecliptic. If this term is reminiscent of eclipse, it is and there’s a reason. If the Moon’s path, for instance, crosses the ecliptic at the point on the ecliptic where the Sun happens to be on that day, then there is a solar eclipse.

The equator on a star chart represents all of the points on the celestial sphere that would be directly over one’s head at all of the equatorial points on the Earth’s surface. If we took the star chart upon which we plotted the ecliptic and taped the left and right edges together with the stars on the inside and the two ends of the equator aligned, we would notice that the ends of the ecliptic are also joined. This is because the ecliptic is also a circle. I should mention that I placed the stars on the inside of the circle, because this is representative of our Earth-bound view from the center of the celestial sphere looking outward. We should note that while the equator stays centered between top and bottom of the circular chart, the ecliptic appears as a sine wave that crosses the equator twice while extending to a peak north of the equator and a trough south of the equator. The reason for the sine form is that the circle of the ecliptic is inclined 23 1/4 degrees to the circle of the equator. And to explain even further, this arrangement of the two circles occurs because the Earth’s axis is inclined to its orbit by 23 1/4 degrees. This was explained to us in grade school as the reason for the seasons that we experience, but few of us are really quite sure why this is so.

There are two points where the ecliptic crosses the equator, and there is a peak and there is a trough that all have special significance with respect to the four seasons. The point at which the ecliptic crosses the equator going from south to north is the Sun’s location on the first day of Spring, or March 21st, give or take a day. This is more properly called the Vernal (Spring) Equinox. The other crossing, where the ecliptic is crossing the equator from north to south marks the Sun’s location on the first day of Fall, or roughly September 21st. This is called the Autumnal Equinox. The point farthest north of the equator represents the Sun’s location on the first day of Summer, or roughly June 21st. This is called the Summer Solstice. And finally, the southernmost point is the Winter Solstice, which occurs on roughly December 21st.

Let’s consider what we know about the four seasons, and then examine how that relates to the ecliptic’s relationship with the equator. We know that the days are longest in the summer and that the Sun is higher in the sky near noon in the summer. Indeed the longest day of the year occurs with the Summer Solstice, which is when the Sun is on the northern most point of the ecliptic, and its rays shine more directly down upon our northern hemisphere location, thus creating Summer’s hot weather. The opposite is true for the Winter. We associate winter with shorter, colder days. The Sun is on the point of the ecliptic that is farthest south of the equator. The days are shorter and the Sun remains low in the sky at noon. The weather is colder because the Sun’s rays shine down on us at a less direct angle. What about the equinoxes? At the equinoxes, the Sun is directly over the equator, and the days and nights are of roughly equal length. The Spring and Fall are associated with the equinoxes, and the weather tends to be milder at this time.

So, we have explored the ecliptic’s relationship to the equator, and how the Sun’s position on the ecliptic is related to the seasons. We’ll save for another month an explanation of how this affects the Moon and planet’s position in the night sky.

Gemini (The twins)

Constellation GeminiGemini, the twins, is a zodiacal constellation that was also one of the 48 constellations cataloged by 2nd century astronomer Ptolemy that remain among the 88 modern constellations. This constellation located above Orion’s left shoulder, and between Taurus and Cancer on the ecliptic. It is highest in the sky at nightfall in March, and is quickly identified by two rather bright stars of approximately equal brightness, Castor and Pollux, that represent each of the Gemini twins. The ecliptic reaches its northernmost separation from the celestial equator in Gemini, and the Sun’s arrival at this point marks the Summer Solstice.  The annual Geminids is a prominent annual meteor shower that peaks between December 13th and 14th. Only one Messier object, M35, is located in Gemini.

IAU Gemini chart, IAU and Sky & Telescope magazine (Roger Sinnott and Rick Fienberg), June 5, 2011.
IAU Gemini chart, IAU and Sky & Telescope magazine (Roger Sinnott and Rick Fienberg), June 5, 2011.

© James R. Johnson, 2014.