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.

The Hercules Cluster

The Hercules Cluster (M13) is located between the two western stars of the Keystone Asterism that represents Hercules’ head. M13 is about 25,100 light years from Earth, and was the target toward which the Arecibo message was beamed in 1974, or 40 years ago. This cluster is comprised of about 300,000 closely packed stars, can be seen with the unaided eye in a very dark sky, and it is easily located with binoculars or modest amateur telescopes. I have not tried, but I think that a camera set up described for the Virgo Cluster could easily capture this cluster.

The Virgo Galaxy Cluster

The distribution of galaxies within the Universe tends to be in clusters. The one nearest our local cluster is the Virgo Galaxy Cluster. It contains about 1500 galaxies, it is roughly circular when viewed from Earth, and its width is about 8 degrees, or about the width of 16 full moons. Like the stars in a star cluster, the galaxies in a cluster are gravitationally related to one another. Because of the cluster’s large size, and because even the brightest galaxies are quite dim, it is not observable at all with the unaided eye, and even the best telescope can see but a portion of the cluster at one time. The best way to observe the cluster is photographically in a wide field of view. Here is my attempt to photograph the cluster last year: http://www.jrjohnson.net/pages/image_template.php?ID=11.

There are about 30 galaxies in this inverse image, and they can be identified with their Messier catalog number (e.g. M86) or their New General Catalog (NGC) number. In a good quality, properly adjusted display, each of the galaxies can be seen as a smudge inside the circle near the M or NGC labels. The source data for this image is 24 full frame, single exposures that were stacked together to reduce the noise (graininess). Each of the individual images were exposed for 30 seconds at f/5.6, ISO 800 using a 70mm lens. Although my camera was guided, an unguided exposure of field this wide should have only very minor star trails, which are caused by the Earth’s rotation during the 30 seconds that the camera shutter was open. The galaxies could be brighter, and perhaps more of them could be seen if a lower f/ number or a higher ISO were used. Experiment with various camera settings, and consult a star chart on line to determine where to point the camera.

Virgo (The virgin)

Constellation VirgoVirgo, the virgin, is one of the zodiacal constellations,  is one of the 48 constellations cataloged by 2nd century astronomer Ptolemy, and is the 2nd largest constellation in the sky. It can be found at is highest point in the sky at nightfall in March. Virgo is located on the ecliptic, flanked by Leo and Libra. It can also be found by following the arc of the Big Dipper’s handle through Arcturus in Bootes to Spica (Alpha Virginis), Virgo’s brightest star. Some of Virgo’s remaining stars can be difficult to see in light-polluted urban skies because this is not a particularly bright constellation. Within Virgo is one of the two points where the ecliptic intersects the celestial equator.  The moment of the Sun’s southward crossing of the celestial equator as it moves along the ecliptic is the Autumnal Equinox, which marks the first day of Fall.

The Virgo Cluster is a very large scale object spanning about eight degrees and containing 1,300 or more individual galaxies. The cluster is centered in Virgo, and extends northward into Coma Berenices. The member galaxies that were cataloged by Charles Messier are M49, M58, M59, M60, M61, M84, M86, M87, M89, and M90. Many other galaxies in this cluster have NGC designations. The Sombrero Galaxy, M104, is a very unusual galaxy in Virgo that is not a member of the Virgo Cluster.

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

© James R. Johnson, 2014.

Hercules (A Roman mythological hero)

HerculesHercules is one of the 88 modern constellations that are among the 48 constellations cataloged by 2nd century astronomer Ptolemy. It is named after the Roman mythological hero adapted from the Greek hero Heracles. It is a northern constellation that lies just off of the Summer Triangle, and near Ophiuchus. Hercules reaches its highest nightfall ascension in July. A squarish asterism called the Keystone is likely to catch the eye first when looking in this area. Older visualizations of this constellation depict the Keystone as Hercules’ hips, while some more modern visualizations depict the Keystone as Hercules’ head.

One of the most notable Messier objects, the Hercules Cluster (M13), is a bright globular star cluster that can be seen with the naked eye in dark skies. It is located between the two stars comprising the Keystone’s western edge. M92 is the only other Messier object in this constellation.  The largest and most massive known structure in the universe, The Hercules-Corona Borealis Great Wall, is a filament of galaxies that is situated on Hercules’ border with Corona Borealis.

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

© James R. Johnson, 2014.

Corona Borealis (The northern crown)

Corona Borealis, BootesCorona Borealis is one of the smallest, but prettiest constellations. It is located between Boötes and Hercules, and is best seen when it reaches its highest nightfall ascension in September. It is also one of the 48 constellations cataloged by 2nd century astronomer Ptolemy. This constellation’s name is inspired by its shape: its main stars form a semicircular arc with Alphecca, a bright jewel, situated about half way around the arc. This distinct constellation easily catches the observers eye in a dark sky, but often escapes notice in a light-polluted urban sky.

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

© James R. Johnson, 2014.

Boötes (The herdsman or plowman)

BootesBoötes is a bright and distinct northern constellation that is one of the 48 constellations cataloged by 2nd century astronomer Ptolemy. It can be found by looking south of Draco, lying between Hercules and Ursa Major. It is also found by following the curve of the Big Dipper’s handle southward and away from the bowl to the bright star Arcturus. Sometimes the phrase “arc to Arcturus” is used to describe this approach to locating it. Boötes reaches its highest nightfall ascension in June. Arcturus is the fourth brightest star in the night sky, and Boötes is home to many other bright stars, including eight above the fourth magnitude and an additional 21 above the fifth magnitude, making a total of 29 stars easily visible to the naked eye.

There are no Messier objects located in Boötes, but several NGC galaxies can be found with a telescope. The radiant of the Quadrantids meteor shower, which displays about 100 meteors per hour when it peaks over January 3rd and 4th, is located between Bootes’ head and the end of Ursa Major’s tail.

IAU Bootes chart, IAU and Sky  Telescope magazine (Roger Sinnott and Rick fienberg), June 4, 2011.

IAU Bootes chart, IAU and Sky & Telescope magazine (Roger Sinnott and Rick Fienberg), June 4, 2011.

© James R. Johnson, 2014.