Category Archives: Astro-Knowlege

Brief descriptions of astronomical concepts needed to understand casual observational astronomy.

Southern Constellations

The southern constellations are those that fall within the Southern Celestial Zone, which is located at the bottom, or southern, portion of the sky map below. This zone is bounded by the celestial equator (0° declination) to the north, and the southern horizon to the south.

Stars in this region tend to hang low in the southern sky,  never rising very far above the horizon. From our N39° latitude on the terrestrial sphere, these constellations generally rise in the southeast, make a brief appearance above the horizon, and then set in the southwest. As a the southern declination of a star increases, the amount of time that it will appear above our horizon decreases.

To gain a sense of how a star’s declination affects its time above the horizon, examine the -20° and -40° declination lines in the chart below. The -20° declination lines spans from the 0th hour (0h) of right ascension line to the 10th hour (10h) of right ascension line, which means that a star traversing the sky at -20° will be above the horizon for ten hours. Now consider an object just 20 degrees farther south on the -40° declination line. This line spans from the 2nd to the 8th hour of right ascension, which means that the star will appear above the horizon for only six hours. The effect accelerates as the declination increases even more. At five degrees farther south, or -25° declination, we approach the southern limit of our view from N39° latitude. An object at this declination would only appear above a perfect southern horizon for just a few minutes.

In contrast, objects that have a more northerly declination remain above the horizon increasingly longer. At +39.14° declination and higher on the celestial sphere, which corresponds to my N39.14° latitude on the terrestrial sphere, objects remain above the horizon all night!


Northern Constellations

Northern Constellations are those that fall within the Northern zone of the celestial sphere, which is zone that spans from east to west across the central area of the sky chart below. This zone is bounded on the north by the circle that defines the Circumpolar region, and on the south by the equator, or 0° declination, that spans from the east to west points on the horizon.

Stars in this zone generally rise in the east, pass directly or nearly directly overhead, and then set in the west. These are the most easily observed constellations when they are on the meridian, because views in this region are the least affected by the atmospheric haze that degrades views near the horizon. This region is also the least likely to be blocked by obstructions such as trees or buildings on the horizon.


© James R. Johnson, 2015.

Cardinal Directions for Casual Stargazers

Compass Rose, Brosen, April 13, 2006. Retrieved from Wikipedia by Jim Johnson on January 19, 2015.
Compass Rose, Brosen, April 13, 2006. Retrieved from Wikipedia by Jim Johnson on January 19, 2015.

Being able to identify the four cardinal directions – north, south, east, and west – is necessary so that stargazers can correctly orient a star chart to permit a desired constellation might be found. Close is good enough for our purpose, and once any one of the cardinal directions is determined, the other three are easily derived by imagining our body as a compass rose. I will describe a compass rose stance and how to use it, and I will present three easy methods to determine at least one cardinal direction. Many more methods can be found by a Web search on “finding cardinal directions without a compass.”

Assume a compass rose stance by standing up, and extending the arms up and straight out from the sides, and horizontal to the ground. Assume that you already know which way is north and that you are facing in that direction. Looking at the compass rose above, we can see that the face is the north point of the rose, the right hand is east, the left is west, and the south point is the back of the head. If we can determine any one of the four cardinal directions, and turn our self until the correct point of the compass rose stance is aligned in that direction, then we can determine the other three cardinal directions off of the the other three points of our compass rose stance.

The easiest method to find one cardinal direction is based on the Sun, and can be used if one knows where the Sun rises and sets. The Sun rises in the east, and it sets in the west. That orange glow on the horizon before sunrise and after sunset also indicates which directions are east and west, respectively. To illustrate, if I am observing in the early evening and can still see the sunset orange glow is brightest at one point on the horizon, I have determined which direction is west. Assuming my compass rose stance with my left hand pointing toward the west, I know that north is straight ahead, east is to my left, and south is behind me.

A compass is just as easy as working of off the sunrise and sunset points if one is available. Since the red arrow will always point north, an observer can face in direction of the red arrow and derive the other three directions by extending the hands toward the horizon. The left hand points west and the right hand points east. South, of course, is behind the observer.

Once north has been determined, it is easier to find Polaris, which can be used in the middle of the night from any location. Read Polaris, The North Star to learn how to find this star and use it to determine which direction is north. I recommend that any serious stargazer invest the time in learning this method.

One of these processes or a similar process must be used to determine the cardinal directions the first time that a stargazer observes from a new location. If a stargazer is to make repeated observations from the same point, then the cardinal directions need only be determined once and committed to memory or noted in an observing log. For instance if an observer remembers that a distinct tree on the distant horizon that is just a little left of North, then using that tree as a future reference, the observer readily knows which direction is north on subsequent visits to that spot.

Knowing the cardinal directions is an inherent part of observing the sky. Learning how to determine these directions is quite easy, and becomes second nature with just a little practice, and after a while the compass rose stance will no longer be needed.

© James R. Johnson, 2015.

Lunar and Solar Eclipses of October 2014

Interestingly, there is both a lunar and a solar eclipse this month. The relationship between these events provides an opportunity gain a deeper understanding of eclipses, and it is an opportunity to explore some characteristics of the Moon’s orbit about the Earth.

Let’s start with the Sun’s role. The Sun’s path among the stars defines the ecliptic. The Sun’s location in the sky, and on the ecliptic can be computed rather precisely for any given date or time. The ecliptic can be found on most star charts. Note that the ecliptic becomes a full 360° circle when the left (west) and right (east) edges of a full sky chart are bent into a cylinder so that the two ends of the ecliptic meet.

A definition of Full and New Moon is essential to understanding solar and lunar eclipses. The Full Moon (the entire face of the Moon is lit) occurs when the Sun and Moon are opposite one another when seen from Earth. In other words, the Earth is located between the Sun and the Moon. The New Moon (none of the face of the Moon is lit) occurs when the Moon is located between the Sun and the Earth. The lit side of the Moon is facing the Sun, and the dark side is facing the Earth.

The Moon’s orbit is inclined to the ecliptic by about 5.5°, which means three things: 1) half of the Moon’s orbit is above the ecliptic, 2) half of the Moon’s orbit is below the ecliptic, and 3) the Moon crosses the ecliptic twice in each orbit. These two points are called nodes. The ascending marks the point at which the Moon crosses the ecliptic headed north, and the descending node marks the south-bound crossing. These nodes progress about the ecliptic once in about 18.6 years, which is why series of lunar and solar eclipses repeat ever 18.6 years.

If the Sun happens to be located at the point of the Moon’s crossing of the ecliptic at the time of the crossing, an eclipse will occur. Since the Sun’s disk (1/2° in diameter) occupies only about 1/720th of the 360° ecliptic, and the Moon may be as much as 5.5° above or below the ecliptic, an eclipse is a rather rare event.

A lunar eclipse occurs when the full Moon passes through the Earth’s shadow. Given that the Earth is between the Sun and Moon at Full Moon, it stands to reason that the Earth’s shadow will fall upon the Moon, if the full Moon happens to be crossing the ecliptic.

A solar eclipse occurs when the new Moon casts its shadow upon the Earth’s surface. This stands to reason given that at New Moon, the Moon is located between the Sun and the Earth. To an observer at a fixed location on the surface of the Earth, the Moon’s dark disk is seen to move across the Sun’s face, either partially, or fully blocking out the Sun at the eclipse’s maximum.

October 8th – Total Lunar Eclipse
This eclipse will begin when the Moon enters the prenumbra (lightest part of the Earth’s shadow) at 4:45am. The Moon enters the umbra (the darkest part of the Earth’s shadow) at 5:15am, and Moon is fully within the umbra (total eclipse) at 6:25am. Unfortunately, the Sun rises and the Moon sets before the eclipse ends.

October 23rd – Partial Solar Eclipse
This month’s solar eclipse is “partial,” because the Moon’s dark disk will not fully cover the face of the Sun. The eclipse will begin when the Moon first begins to cover the Sun’s face at 5:52pm, and it will reach its maximum coverage of the Sun’s face at 6:17pm, which is sunset.

The Circumpolar Region of the Celestial Sphere

A circumpolar object (star, constellation, deep space object) is one that never sets below the horizon during the Earth’s daily rotation. Any point in the sky that has a declination (degrees of separation from the celestial equator) greater than the observer’s latitude (degrees of separation from the terrestrial equator) will never set below the horizon. If for some reason the Sun ceased to illuminate the sky, a circumpolar object could be seen to circle Polaris once in about every 24 hour period. Cepheus, for instance, is a circumpolar constellation that is oriented with the top of the house-like asterism nearest Polaris. In the September evening sky, Cepheus is high above Polaris, and the “house” appears to be upside down. Over the course of 24 hours, Cepheus will circle Polaris. In six hours, the “house” is horizontal with the top pointing west, in 12 hours it appears upright, but below Polaris, in 18 hours it appears vertical again, this time with the top pointing east. Six months from now in March, Cepheus’ orientation at dusk is the same upright appearance as Cepheus’ 12-hour position in September.

The circumpolar region of the celestial sphere is indicated on the annotated sky map below as the nearly circular region. Two points define this region, the north celestial pole, and the north point on the horizon. The north celestial pole is at the center of the region’s circular border, and all stars on the map circle around this central point as a result of the earth’s daily rotation about its axis. Stars near the pole make small tight circles around the pole, while those out near the drawn circle, but still within the circle, make wider sweeping circles. Those stars outside of the circle also circle the pole, but will dip below the the northern horizon, and are therefore not circumpolar stars.


© James R. Johnson, 2015.

The Speed of Light

Light travels in photons that are either emitted from hot objects like light bulb filaments, or reflected off of cooler objects, like the walls. The Sun and Moon are also emission and reflection objects, respectively. Photons travel from their source to our eye or camera sensor at, well, the speed of light. We can express this speed in terms with which we are already accustomed, like miles per hour (mph). Since light travels at 186,282 miles per second, that equates to 670.6 million mph. That is incredibly fast! Since mph has a distance component (miles), we can use the speed of light, or the speed of anything else, to measure distance. If something is 670.6 million miles away, we can also say that it is one light-hour away.

The amount of time involved essentially becomes the yardstick. The Moon is about two light-seconds away. The Sun is eight light-minutes away. A light-hour does not get us all the way to Saturn, and the closest star is four light-years away. Galactic sizes are on the scale of hundreds of thousands of light-years, and intergalactic distances are on the scale of millions and even billions of light-years. There’s nothing like the distances across the vast emptiness of space to make light seem to flow like molasses.

An implication of light’s incredibly slow speed is that our eyes are essentially time machines. The point at which Saturn appears in the sky is actually where it was over an hour ago. Similarly, The Andromeda Galaxy, our closest galactic neighbor, is seen as it was 2.5 million years ago. A supernova detected today in a galaxy located 11 million light-years away actually happened 11 million years ago.

Aphelion – July 24, 2014

The Earth reaches aphelion (Greek apo [away from] + Helios [Greek god of the Sun]) on July 24th. Aphelion is the point in the Earth’s orbit that is farthest, or 94,555,000 miles from the Sun. Relatedly, perihelion (Greek peri [around] + Helios [Greek god of the Sun] is the point in the Earth’s orbit that is closest to the Sun, or 91,445,000 miles from the Sun.

Aphelion and perihelion apply not only to the Earth’s orbit about the Sun, but any object that orbits the Sun to include all of the planets, asteroids, comets, and even man-made satellites in solar orbit. Apogee and perigee are similar terms for objects that orbit the Earth, which are either the Moon or man-made satellites.

Aphelion and perihelion points are not only opposites that describe the Earth’s farthest and closest distance to the Sun during each annual orbit, these two points are on opposite sides of the Sun. Aphelion occurs July 24th and perihelion occurs on January 4th. Although the aphelion and perihelion points are on exactly opposite sides of the Sun from one another, the dates are not exactly six months apart. This is because the Earth moves more slowly during the perihelion to aphelion (up hill) part of its orbit, and faster while during the aphelion to perihelion (down hill) part of its orbit.

We learned in grade school that the Earth’s average distance to the Sun is 93 million miles. Those who remember that fact at all are likely, if asked, to omit “average,” and state that the Sun is 93 million miles away. If the Earth varies in distance to the Sun from 94.5 to 91.4 million miles over the course of each orbit, then that accounts for the average figure of 93 million miles that is often cited.

Let’s explore why the Earth’s distance to the Sun varies, and what the implications are of that variance. If the Earth were in a perfectly circular orbit, then its distance from the Sun would not vary. In this case, 93 million miles might be the constant distance to the Sun throughout each annual orbit. Like the orbits of most objects around a parent body, the Earth’s orbit is not circular, but is elliptical. An ellipse resembles a stretched or flattened circle, and an object following this type of orbit will vary between its closest and farthest points once per orbit.

The Earth’s elliptical orbit has implications for solar eclipses. Both the Sun and the Moon are said to be about 1/2 degree in angular diameter as observed from Earth. This is because the actual diameter of both or coincidentally 400 times their diameter, and it accounts for the near-perfect fit when the Moon barely covers the face of the Sun during solar eclipses. The Sun’s apparent size when seen from Earth varies inversely with respect to Earth-Sun distance as the Earth moves from aphelion to perihelion. The Sun appears smaller when viewed from Earth at aphelion, and larger at perihelion. Similarly, the Moon appears smaller at apogee and larger at perigee. Consider the effect if the Earth were at perihelion and the Moon were at perigee at the time of a solar eclipse. The Sun would be at its smallest possible apparent diameter and the Moon would be at its largest possible apparent diameter. The result would be a total Solar eclipse where the larger Moon completely obscures the face of the smaller Sun. Now consider the opposite case. The smallest possible Moon at apogee would not be able to completely obscure the face of the largest possible Sun at perihelion. The result would be an annular Solar eclipse where the Sun would appear as a ring around the Moon.

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:

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.

The Inclination of Saturn’s Rings

As Saturn cycles through its 29 1/2 year orbit about the Sun, we on Earth will alternatively see the top (north face) of Saturn’s rings for roughly half of its orbit, and the bottom half (south face) of Saturn’s rings for the other half. This occurs as a result of how the rings are inclined with respect to the plane of Saturn’s orbit. As the view transitions between upper and lower halves of the rings, our view will be edge on, and the rings will seem to have temporarily vanished. The last edge-on view was in 2009, at which time our current view of the north face of the rings began opening up. Even before Saturn’s rings fully open in 2017, our present view is pretty spectacular.

Polaris (The North Star)

Circumpolar star trailsPolaris’ special significance is that it is the Pole Star, located very near the point at which the Earth’s northern axis intersects the celestial sphere. As a result of this unique location in the sky, Polaris will appear to remain stationary all night while all of the other stars will appear to rotate around it. You may have seen star trail images that illustrate this effect. Another implication of Polaris’ unique location is that it is a measure of one’s latitude. From Ashton, MD for example, Polaris appears 39.15° above the point on the horizon in the direction of due north, which corresponds to Ashton’s latitude on a map or globe. Polaris marks the end of the bear’s tail in Ursa Minor, or the end of the handle of the Little Dipper.

The best place to start when looking for Polaris is the Big Dipper which is part of the constellation known as Ursa Major. The two stars at the end of the bowl farthest from the handle are pointer stars. Follow an imaginary line from the pointer star at the  bottom of the bowl, through the one at the top of the bowl. The 2nd magnitude star that is about five times the distance between the two pointer stars is Polaris.

The Big Dipper Pointer Stars, Jim Johnson, December 27, 2014.
The Big Dipper Pointer Stars, Jim Johnson, December 27, 2014.

Polaris has not always been the Earth’s Pole Star, nor will it always be the pole star. As a result of the procession of the equinox, the axis about which the Earth rotates will trace out a large circle every 26,000 years, as illustrated below. This sounds like a long time, but when the Egyptian pyramids were built 5,000 years ago, Thuban in Draco was the pole star, and thus some north facing entrances of the pyramids were aligned on this star. In 8,000 years from now, Deneb in Cygnus will be the pole star, and the Earth’s axis will be aligned with a point near Vega in about 12,000 years from now.

© James R. Johnson, 2014.