Tag Archives: Ecliptic

Aries (The ram)

Constellation AriesAries is the zodiacal constellation named after the ram.  In ancient times, this constellation represented the golden fleece that was stolen by Jason and the Argonauts. It is one of the 48 constellations cataloged by 2nd century astronomer Ptolemy that remain among the 88 modern constellations. It is located on the ecliptic between Pisces and Taurus, and just south of Triangulum. It is highest in the sky at darkness and can be best seen in December.  Its brightest star is Hamal (Alpha Arietis), and most of this constellation’s stars can be picked out in most light polluted urban skies, but the constellation bears little resemblance to a ram.
http://en.wikipedia.org/wiki/Aries_(constellation)

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

© James R. Johnson, 2014
jim@jrjohnson.net

Cetus (A sea monster in Greek mythology)

CetusCetus, the Greek mythological sea monster, is a large southern constellation on the celestial equator, and one of the 48 constellations cataloged by 2nd century astronomer Ptolemy. It is accompanied by other water constellations – Pisces, Aquarius and Eridanus. Its head is nestled among three zodiacal constellations: Taurus, Aries and Pisces. Its body is south (below) Pisces, and is flanked by Aquarius and Eridanus. Cetus’ close proximity to the ecliptic means that the Moon, planets and asteroids occasionally pass through this constellation. Cetus is placed highest in the sky at nightfall in December.

Cetus is home to one Messier object, M77, a striking face-on spiral galaxy. Among Cetus’ stars is an unusual variable star, the disappearing  one, or Mira (Omicron Ceti). This star’s high variability from 3rd magnitude to 10th magnitude means that it disappears from view when it dips below 5th or 6th magnitude. Cetus’ brightest star Menkar (Alpha Ceti) marks the sea monster’s nose.
http://en.wikipedia.org/wiki/Cetus

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

© James R. Johnson, 2014.
jim@jrjohnson.net

Pisces (The fishes)

Constellation PiscesThis zodiacal constellation is a rather dim grouping of stars that are strung out low across the southern sky, just below the Great Square of Pegasus. Pisces is one of the 48 constellations cataloged by the 2nd century astronomer Ptolemy that remain among the 88 modern constellations. It is highest in the sky and best observed at nightfall in December. The intersection of the ecliptic and the celestial equator lie within this constellation. There are two such intersections that mark the two equinoxes. At the intersection in Pisces, the Sun is moving from south to north, so this is the point of the Vernal (spring) Equinox. This constellation is home to just one Messier object, M74.
http://en.wikipedia.org/wiki/Pisces_(constellation)

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

© James R. Johnson, 2014
jim@jrjohnson.net

Aquarius (The water bearer)

Constellation AquariusAquarius, the water bearer, is a rather dim and nondescript zodiacal constellation. This southern constellation was cataloged by 2nd century astronomer Ptolemy. It is located in a region known to  the ancients as “the sea” because several water constellations, to include Capricornus (the sea goat), Piscis Austrinis (the fish), Pisces (the fishes), and Cetus (the sea monster) are gathered there. It is situated on the ecliptic between Capricornus and Pisces, and is just south of the Great Square of Pegasus. Aquarius reaches its highest nightfall ascension in November. Three globular star clusters, M2, M72 and M73 are the only Messier objects found in this constellation. The Eta Aquariids is the strongest meteor shower radiating from Aquarius, peaking at about 35 meteors per hour between May 5th and 6th.
http://en.wikipedia.org/wiki/Aquarius_(constellation)

IAU Aquarius chart, Sky & Telescope magazine (Roger Sinnott and Rick Fienberg), January 27, 1997.
IAU Aquarius chart, Sky & Telescope magazine (Roger Sinnott and Rick Fienberg), January 27, 1997.

© James R. Johnson, 2014.
jim@jrjohnson.net

Capricornus (The sea goat)

Constellation CapricornusCapricornus, the sea goat, is a zodiacal constellation and one of the 48 constellations cataloged by Ptolemy in the 2nd century. It is located on the ecliptic between Sagittarius and Aquarius, and is south of the Summer Triangle and the Great square of Pegasus.  Capricornus reaches its highest nightfall ascension in October. Its alpha star, Deneb Algedi (Delta Capricorni), is just brighter than magnitude 3, and can be seen under most reasonable conditions. The remainder of the constellation is rather dim and difficult to see in light-polluted urban skies. The globular star cluster, M30, is the only Messier object in this constellation.
http://en.wikipedia.org/wiki/Capricornus

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

© James R. Johnson, 2014.
jim@jrjohnson.net

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.
http://en.wikipedia.org/wiki/Sagittarius_(constellation)

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.
jim@jrjohnson.net

Scorpius (The scorpion)

Constellation ScorpiusScorpius is perhaps the most beautiful of the zodiacal constellations. This southern constellation is located on the ecliptic between Libra and Sagittarius and is situated south of Ophiuchus. Antares (Alpha Scorpii), the reddish 1st magnitude star might first catch the observers attention. Then look for the remainder of the constellation which includes a distinct head, long slender body, curling tail, and stinger of a scorpion. Even though Sagittarius is on the lowest point of the ecliptic, Scorpius is the southernmost of the zodiacal constellations, which tends to leave the tail hanging below the treeline for us northern latitude observers. Southbound travelers in July, which is when Scorpius reaches its highest nightfall ascension, should make it a point to observe this very pretty constellation.

Scorpius’ location on the Milky way makes it home for many deep sky objects, such as the Butterfly Cluster (M6) and the Ptolemy Cluster (M7). Other objects cataloged by Charles Messier include globular clusters M4 and M80.
http://en.wikipedia.org/wiki/Scorpius

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

© James R. Johnson, 2014
jim@jrjohnson.net

Libra (The scales)

Constellation LibraLibra, the scales, is a small zodiacal constellation of  rather dim stars that was cataloged by 2nd century astronomer Ptolemy. It is located on the ecliptic between Virgo and Scorpius, and is to the south of Ophiuchus. This southern constellation, which never rises very far above the horizon, reaches its highest nightfall ascension in June. Its brightest star, Zubenelgenubi, is just brighter then 3rd magnitude, which can be a challenge to see in some light-polluted urban skies.
http://en.wikipedia.org/wiki/Libra_(constellation)

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

© James R. Johnson, 2014.
jim@jrjohnson.net

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.

Leo (The lion)

Constellation LeoLeo, the lion, is a distinctive zodiacal constellation that was cataloged by 2nd century astronomer Ptolemy. It is located on the ecliptic between Cancer and Virgo, and can be found below the bowl of the Big Dipper with both constellations arriving at their highest nightfall ascension in April.  Leo closely resembles a crouching lion with its rather distinct backward question mark, or sickle asterism. This constellation features the bright star Regulus (Alpha Leonis) at the back of the lion’s head.

The most famous deep sky object in Leo is The Leo Triplet is a close grouping of the galaxies M65, M66 (displayed) and NGC3628 that can be seen together in a single telescopic field of view. For those observing with telescopes, there are several other galaxies cataloged by Charles Messier are located in Leo, to include M65, M66, M95, M96 and M105. Leo is also home to the famous Leonids meteor shower that occurs during November, peaking at about 10 meteors per hour on November 14th and 15th.
http://en.wikipedia.org/wiki/Leo_(constellation)

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

© James R. Johnson, 2014.
jim@jrjohnson.net