As the Earth continues its orbital journey about the Sun, the stars that are visible at a particular time each evening will have moved westward (toward the sunset) when observed the same time the next day. For instance, every star that you see at 9pm this evening will have shifted slightly toward the sunset point when you observe again at the same time the next evening. Over the course of a week or a month, this change can be pretty dramatic. Orion, for example, was prominent at sunset in April, but has shifted so far toward the Sun that it is now lost in the Sun’s glare before sunset and cannot be seen in the evening sky during June. My approach to the Constellations section of Scope Out is to cover the most important constellations that have come into view from the east over the past month.
This month will we will identify three constellations that were among the 48 constellations described by the 2nd century astronomer Ptolemy and remain among the 88 modern constellations: Boötes, Corona Borealis, and Hercules. The reason that Ptolemy’s constellation charts have only half the number that are recognized today is that Ptolemy never travelled to the southern hemisphere. I include one zodiacal constellation in this month’s edition, and since one new zodiacal constellation will have advanced to “center stage” (the meridian just after sunset) each month, I will highlight one zodiacal constellation each month. Be sure to visit April’s Scope Out for a brief description of aids that can help locate a constellation and identify its stars.
Boötes (The Herdsman or Plowman)
First up this month is Boötes, which is 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. It is one of the brightest, prettiest constellations. 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.
Corona Borealis (Northern Crown)
Corona Borealis is a small constellation in the northern sky, just east or to the left of Boötes. This constellation’s name is inspired by its shape: its main stars form a semicircular arc with a bright jewel near the center. I was fascinated by this very distinct constellation when I first learned the constellations as a teenager. It would always catch my eye in the summer sky, and I eventually consulted a star chart to learn its name. The Crown is not nearly as noticeable in today’s suburban light-polluted skies where I live.
Hercules is a constellation named after the Roman mythological hero adapted from the Greek hero Heracles. It can be found just east (to the left) of Corona Borealis and Boötes. 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. I have always “seen” the latter. There is a magnificent star cluster (M13) located between the two stars comprising the Keystone’s western edge.
Virgo (The Virgin)
Next is Virgo, the zodiacal constellation that follows Leo, which means that it can be located to the left, or east of Leo. It can also be found by following the arc of the Big Dipper’s handle through Arcturus to Spica, Virgo’s brightest star. Some of Virgo’s remaining stars can be difficult to see because this is not a particularly bright constellation, and because the constellation does not rise as high in the sky as the previous zodiacal constellations that have been covered thus far in Scope Out. This is because Virgo is located in the autumnal section of the ecliptic, meaning that by the time autumn rolls around, the Sun will be located in Virgo. As the Sun moves through this section of the ecliptic, the days will be getting shorter and the Sun will not rise as high in the sky at mid-day. Mars is presently located in Virgo.
SOLAR SYSTEM OBJECTS
The Sun reaches summer solstice on June 21st. This day is the first day of summer and the longest day of the year. From an astronomical perspective, the Sun is on the point of the ecliptic that is farthest north of the equator. From a cartographic perspective, the Sun is on the Tropic of Cancer, which is about 23.25 degrees north of the equator. Someone standing on the Tropic of Cancer on this day will see the Sun pass directly overhead at noon.
|June 5||First Quarter|
|June 7||Conjunction with Mars|
|June 10||Conjunction with Saturn|
|June 13||Full Moon|
|June 19||Last Quarter|
|June 24||Conjunction with Venus|
|June 27||New Moon|
Venus remains a morning star throughout the month. It is the brightest object in the eastern sky in the pre-dawn hours, above and to the right of the sunrise point. It will slowly decrease in brightness as its elongation from the Sun continues to decrease.
Mars remains in Virgo, and is already well above the horizon at sunset. As this planet is past opposition, Earth is speeding past Mars, and Mars begins to dim as the apparent size of its disk decreases. Telescopic views will be better earlier in the month.
Jupiter‘s position at sunset will be moving closer to the western horizon each evening as the month progresses, and the ability to observe Jupiter in the evening will soon be lost as its orbit takes it to the far side of the Sun as seen from Earth. In a few months, Jupiter will appear on the other side of the Sun as a morning object.
Saturn, the ringed planet, viewed through a telescope is the most breathtaking of all the planets. It can be easily found because it is located in the constellation Libra, where it stands out as the brightest object in a region with very few bright stars. Saturn will be well placed for viewing in the east.
DEEP SPACE OBJECTS (DSOs)
So far, Scope Out has examined nebulae, star clusters and galaxies in this section. Are there larger cosmic structures than galaxys? The Universe is larger than a galaxy. Is the Universe a collection of galaxys, or is there something larger than a galaxy, yet smaller than the Universe?
First this month we will consider the Virgo Galaxy Cluster. The distribution of galaxies within the Universe tends to be in clusters, and this is the galaxy cluster nearest Earth. 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. 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.
Next, lets locate another star cluster, perhaps the most beautiful one of all. 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 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.
© 2014 James R. Johnson.