Thursday, March 27, 2008

Secrets of the Zodiacal Light

Sometimes the great adventure of observational astronomy feels like an initiation into a secret society. You learn arcane preparatory rites like dark adapting, you learn to read with a red flashlight, and you learn about celestial objects and phenomena that were within your visual grasp all along, hiding in plain sight.

One example of the latter is the zodiacal light. If you’re a beginning stargazer, this may be an unfamiliar term. But even if you know what it is and what it’s supposed to look like, it can still elude you. The zodiacal light is in the observing category I call “Things That Are Barely There.”

The zodiacal light (pronounced zoh DYE uh cull) is a large, roughly triangular or cone-shaped glow in the night sky, extending upwards from either the eastern or western horizon. The base of the triangle or cone--the widest part of the glow--rests on the horizon. The zodiacal light is typically seen above the western horizon after sunset and above the eastern horizon before sunrise.

There are several reasons why the zodiacal light is somewhat elusive:

a) Its glow is easily confused with the glow of twilight. Twilight is the time before sunrise and after sunset when the Sun, although below the horizon, lights up Earth’s upper atmosphere. Some of that light is reflected toward Earth’s surface, creating a condition of faint illumination.

Avoid confusion with twilight by only trying for the zodiacal light before morning twilight or after evening twilight. To find out when twilight begins and ends at your location on a given date, use Form A at this U.S. Naval Observatory link. Under “Type of Table,” select “astronomical twilight.” Enter your location information, and then click the Compute Table button.

b) Its visibility is dependent upon good transparency. Transparency means atmospheric clarity. Clouds, haze, dust, or humidity can cause poor transparency. The glow of the zodiacal light is subtle, so it's best to have a crisp-looking sky, with good contrast between the bright stars and the deep blackness that surrounds them.

Avoid trying for the zodiacal light when transparency is poor, for example, if a big windstorm just blew through your area. A milky-looking sky makes spotting the cone of light difficult to impossible. In addition, it's critical to be in a dark-sky site, away from urban and suburban light pollution that degrades sky contrast. Finally, you need to avoid times when the Moon is up.

c) It is best seen at specific times of the year. In the Northern Hemisphere, the visibility of the zodiacal light varies greatly throughout the year, but it's most prominent when the ecliptic is nearly perpendicular to the western or eastern horizon. Some good times to view it are February and March after sunset, and September and October before sunrise.

The ecliptic is the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth. Because the Earth, Moon, and planets all lie in roughly the same plane as they orbit the Sun, the ecliptic can also be said to represent the plane of the solar system.

The zodiacal light is so named because it aligns with the ecliptic, which coincides with the zodiac. The zodiac is the band formed by the twelve constellations that straddle the ecliptic, as seen from Earth. To understand why the zodiacal light coincides with the ecliptic, we must unlock the secret of its mysterious glow.

Interplanetary space is not empty. Our solar system contains a vast number of minute particles, some left over from the time of planet formation and some ejected from passing comets or asteroid collisions. Light from our Sun illuminates the myriad particles lying in the solar system plane, so we see the reflected sunlight as a luminous pillar of light along the ecliptic. If you're fortunate enough to view the zodiacal light when transparency is good and the sky is inky black, you may notice the light pillar has a yellowish cast. Our Sun is, after all, a yellow star, so it shines with a golden light.

Here are some links to good photos of the zodiacal light:
http://www.astrophoto.com/ZodiacalLight.htm
http://www.nsf.gov/news/mmg/media/images/fig2_sm2_h.jpg
http://www.allthesky.com/various/bz26zodiac28.html

This is a great week to search for the zodiacal light after sunset, so let’s head out to the darkest spot we can find.

1) The zodiacal light should be visible about an hour and a half after sunset. The Moon doesn't rise until after midnight on the evening of Thursday, March 27. After that, it rises later each night. So it won't interfere with your quest.

2) Face west, the direction where the Sun set.


3) Look for a very large, faint cone of light rising from the western horizon high into the sky. It will slant slightly toward the southeast, following the curve of the ecliptic. If it's a clear night and you don’t see it, try this tip. Look toward the southwest and locate the constellations Orion and Canis Major. Examine this area of the sky and notice how black the space between the stars is. Now turn back to the west and look at the sky there. Do you see a difference between the blackness of the Orion/Canis Major region and the blackness of the region above the western horizon? The western sky should appear more milky. If you see this difference, try to define the boundaries of the milkiness. You should begin to see the pillar of the zodiacal light emerge.

If at first you don’t succeed, gaze gaze again. With a phenomenon so dependent on sky conditions, you may want to make spotting it a long-term goal. I’ve historically had a hard time seeing the zodiacal light; I really have to work at it. However, just last month I saw my most spectacular zodiacal light ever. Transparency and contrast were superb, and the cone of light was so obvious it was like a smack in the face.

Thank you, may I have another?

Thursday, March 20, 2008

Man in the Moon

Have you ever seen the Man in the Moon? The Woman in the Moon? The Rabbit in the Moon? They are there, if you can muster a bit of imagination and whimsy.

I've read somewhere that we Earthlings have a strong impulse to personify the universe. Perhaps our need to see galloping horses, ice cream cones, and the profile of our Aunt Hazel in the clouds is the same as our need to recognize facial features on a lifeless chunk of rock a quarter of a million miles away.

Seeing figures on the Moon has been a common practice in many cultures throughout history. These figures are examples of pareidolia (pronounced pear eye DOH lee uh), the human practice of attributing patterns or meaning to random audio or visual events. Other examples of pareidolia are the “Face on Mars” and the “Jesus Tortilla.”



Full Moon is considered the worst time to look at the Moon’s features through binoculars or a telescope, because surface shadows are at a minimum. It is the play of sunlight and shadow on the Moon’s battered topography of craters, ridges, and valleys that brings those features into high relief and makes them satisfying to observe.

However, Full Moon and a few days on either side is a great time to look for pareidolia. Full Moon occurs on Friday, March 21, so let’s catch a little full moon fever, shall we?




1) The Full Moon is in the sky all night. It rises at sunset and sets at sunrise. You could also say that, during Full Moon, the moon is at opposition. This means it is directly opposite the Sun with respect to Earth’s position, with Earth in the middle. After the 21st, the Moon will rise after sunset, progressively later each night. In addition, the illuminated area on the Moon’s face will wane (shrink) a little more each night.


Clockwise from top left:
Full Moon, Woman, Rabbit, Man in the Moon
Image from Wikipedia

2) Maria, the dark circular splotches on the Moon, are critical components of the Moon’s pareidolia. Maria (pronounced MAH ree yuh) is the Latin word for seas. The singular of the word is mare (pronounced MAH ray). A mare is a crater that filled with lava that afterwards cooled and solidified into basalt. These expanses of basalt account for the smooth, dark appearance of the Moon’s maria, which are hundreds of miles in diameter.

The Moon’s craters are scars from its youth, when it was bombarded by meteorites (rocky space debris that impacts another body). The Earth’s atmosphere protects our planet from meteorites--the friction created on entry vaporizes them so we see them as meteors, aka shooting stars. However, the Moon has no atmosphere, so it took quite a pummeling.

The ‘seas’ and some other lunar surface features bear romantic names. Many were named by Giovanni Riccioli, an Italian Jesuit who published a lunar map in 1651.

3) Using the image above, try to spot the eyes, nose, and mouth of the Man in the Moon. The figure's eyes are the Sea of Showers (Latin name: Mare Imbrium) and the Sea of Tranquility (Mare Tranquillitatis). The nose is the Bay of Tides (Sinus Aestuum). Its gaping mouth is composed of the Sea of Clouds (Mare Nubium) and the Sea that Has Become Known (Mare Cognitum). Note that the Moon’s left eye, the Sea of Tranquility, is the site of the first manned Moon landing in 1969.

4) Now try to spot the Woman in the Moon. Seeing a woman’s face on the moon links us to a widespread cultural tradition. The ancient Greek goddesses Selene and Artemis were both associated with the Moon. The Chinese goddess Chang’e lived on the Moon after she swallowed too much elixir of immortality. One of the two prominent deities of the Maasai people of Kenya and Tanzania is the moon goddess Olapa. There are many more examples worldwide of religious, mythological, and culturally symbolic female figures linked with the Moon.

5) Can you snare the Rabbit in the Moon? There are many allusions in world folklore to a Moon-rabbit connection, although the origin of this association is unclear. A number of Asian and African cultures link the Moon with a rabbit or hare. A Hindu word for the Moon means marked with the hare. Could it be as simple as this: the rabbit is a widespread species, its ears give it an immediately recognizable form, and many people can easily imagine the silhouette of a rabbit in the Moon’s markings? You decide.

6) Back in the old U.S. of A., March’s Full Moon is called Worm Moon by the Algonquins, Wind Strong Moon by the Taos Pueblo Indians, and Moon when the Juice Drips from the Trees by the Delaware Indians.

All signs of spring, to be sure. Happy Equinox.

Thursday, March 13, 2008

The Equinox and the Equilux

If you’re familiar with the word “equinox,” but unsure of its meaning, you're probably in the majority. Although an astronomy term, it has found its way into the vernacular--perhaps because it manages to sound simultaneously technical and mystical. The word has been used to brand such diverse offerings as an SUV from Chevrolet, a classic rock album by the band Styx, and a national chain of fitness clubs.

Pronounced either EE kwi nocks or EH kwi nocks, the word is derived from a Latin word meaning equal night, the significance of which we'll examine in a minute. An equinox is the moment in time, calculated to the minute, when the center of the Sun is above the Earth’s equator. This occurs only twice a year: in the spring around March 20 and in the fall around September 22-23. The Spring Equinox, also known as the Vernal Equinox, conventionally marks the official first day of spring in the United States. This year the Spring Equinox occurs on Thursday, March 20 at 1:48 a.m. Eastern Daylight Time (adjust as needed for your time zone). In the Mountain and Pacific time zones, because of the time difference, the Spring Equinox will actually fall on Wednesday, March 19, but calendars show the equinox and the first day of spring on March 20.

To understand why equinoxes occur only twice a year, you need to remember that Earth’s axis is tilted 23.5 degrees with respect to its orbit around the Sun. If it were not, then Earth’s equator would line up with the plane of Earth’s orbit around the Sun, and the center of the Sun would always be above the equator. Of course, we would have no seasons. The temperatures at a given location on the globe would not vary much over the course of a year. We would not see the Sun arcing high in the sky in the summer or arcing low--nearer the southern horizon--in the winter. It would pretty much follow the same course in the sky, sunrise to sunset, day after day, all year long. Wouldn’t that be boring!


Earth’s tilt with respect to its orbit around the Sun
Image courtesy of Tau’olunga

But don't worry. Our 23.5-degrees-off-kilter planet ensures that here in the Northern Hemisphere we tilt toward the Sun in the summer and experience the heating effect of direct sunlight, and then tilt away from the Sun in the winter and get the chilling effect of indirect sunlight. In spring and fall, midway between the two temperature range (and tilt) extremes, we enjoy moderate temperatures.

Another way to think of the equinox is that it's when the ecliptic (the imaginary line that represents the path the Sun appears to take across the sky, as seen from Earth) intersects with the celestial equator (the imaginary line that represents where the plane of the Earth’s equator, were it to be extended out into space, would intersect with the sky). The diagram below shows the twice-yearly intersection of these two lines, with the Vernal or Spring Equinox marked. The other point of intersection shown corresponds to the Fall Equinox. The celestial equator is shown in blue and the ecliptic in red.


A Tale of Two Planes: Equator & Ecliptic
Diagram by
Dr. Guy Worthey


The Earth’s sunward tilt in the summer puts the plane of the equator below the Sun’s center (which corresponds to the ecliptic), and the Earth’s sun-shunning tilt in the winter puts the plane of the equator above the Sun's center. Only during the in-between seasons of Earth’s orbit--spring and fall--can the Sun’s center intersect with the celestial equator and therefore be directly above the Earth’s equator.

The equal night reference from the Latin word origin refers to the fact that on the dates when the equinoxes occur, day and night are of approximately equal length everywhere on Earth. Note I said “approximately.” You must in fact identify the Spring Equilux if you wish to identify one of the two dates during the year when day and night are closest to being equal at your particular location. The Fall Equilux is the other date. The equiluxes occur quite close to the equinoxes; the Spring Equilux occurs a little before the Spring Equinox. Equilux, pronounced EE kwi lucks, means equal light.

The easiest way to pinpoint your Spring Equilux is to use the indispensable Sun and Moon Data calculator. Enter the date on which the Spring Equinox falls in your time zone, enter your location information, and click the Get Data button. Note the sunrise and sunset times, and then calculate the difference between them in minutes (for example, 7:18 p.m. sunset and 7:11 a.m. sunrise = difference of 7 minutes). This will be the time to beat. Now enter the date right before the first date you entered and check the difference in minutes between sunrise and sunset. Keep working your way back, one day at a time, until you identify the date with the smallest difference, that is, the date when sunrise and sunset are closest to being exactly 12 hours apart. Then you will know when to celebrate your Spring Equilux.

In my location in the Mountain time zone, the Spring Equinox falls on March 19. My Spring Equilux will fall on March 16, when sunrise and sunset will be exactly 12 hours apart. I rather like the balance of yin and yang embodied by the equilux. I think I’ll celebrate by consuming equal amounts of white and dark chocolate. For good measure, maybe I’ll burn a candle at both ends too.



Chinese symbol of yin and yang (dark and light)

Thursday, March 6, 2008

The Winter Hexagon

If you’re a regular reader of my blog, then you know I’m a fan of using asterisms (recognizable star patterns) rather than constellation figures to navigate the night sky. This method is widely used in the amateur astronomy community, and it works.

Most amateur astronomers couldn’t tell you off the top of their heads where one constellation ends and another begins. They couldn’t tell you precisely where the official constellation boundaries lie, that is, the ones designated by the International Astronomical Union in 1930. And they probably couldn’t tell you how the stars of a particular constellation connect in order to resemble a charioteer, a winged horse, or a whale. Now, I have my own theory that quaffing a large quantity of high-octane mead is required to see the star pictures that the ancients saw, but I digress.

What amateur astronomers can tell you is how to spot the Sickle, the Great Square, the Teapot, and the Lazy W--and in which constellation each is. In other words, amateurs use asterisms as celestial landmarks to let them know which 'country' they're in, even if they couldn’t tell you precisely when they crossed the border.

This week I invite you to locate and explore a unique asterism--one that does not lie within a constellation, but rather spans six constellations. Learning it will help you master the winter sky. This marvel is called the Winter Hexagon.


The Winter Hexagon stars (marked in yellow)


Pull on your long johns and your bunny boots, and let’s head out.

1) You’ll need to face south, so if you don’t know the cardinal directions at your location and you don’t have a compass, make note of where the sun sets on the horizon. That spot is approximately west. Stand with your right shoulder to the west, and you’ll be facing approximately south.

2) Wait at least one hour after sunset to begin observing, so your sky is good and dark. New Moon--when the moon does not appear at all in the night sky--occurs on Friday the 7th, so the weekend through Monday night would be the best time to hunt the Hexagon without interference from moonlight. After that, you would have to wait too long for the waxing (growing) crescent moon to set, and the Hexagon stars would begin setting also.

3) I use a memory prompt to remember the six stars that make up the Winter Hexagon. It is CARS-PP, that is, the word “CARS” followed by two P’s. I can remember this easily, and it gives me the first letter of each of the six stars, in order as they are positioned around the Hexagon. The stars are Capella, Aldebaran, Rigel, Sirius, Procyon, and Pollux. All six are bright, easily-seen stars, and each is in a different constellation.

Feel free to start anywhere you like on the Hexagon. Use the star map at the top of this post as your guide. You will find all six constellations somewhere between the zenith (the point directly above your head) and the southern horizon. If you’re a regular reader, you'll recognize a number of these constellations from previous posts. You may find that you recognize certain constellations more easily than others, so you may want to leapfrog around the Hexagon until you are sure you have located all six stars. If nothing else, CARS-PP will simply ensure you don’t forget one.


4) Capella is the brightest star in Auriga the Charioteer. Capella (pronounced cah PELL uh) is actually a binary with two yellow component stars orbiting each other. We see their combined light as one star. Auriga’s defining asterism is a pentagon-like shape, and it connects with the sharp end of Taurus the Bull’s upper horn.

5) Aldebaran is the brightest star in Taurus the Bull. Aldebaran (pronounced ahl DEBB er ahn) is an orange giant star found in the distinctive “V” asterism that outlines the bull’s face. Aldebaran marks the position of the bull’s eye.


6) Rigel is the brightest star in Orion the Hunter. Rigel (pronounced RYE jull) is a blue supergiant. It can be found at the lower right corner of the large, hourglass-shaped asterism that defines Orion.

7) Sirius is the brightest star in Canis Major the Big Dog, and it is also the brightest star in the night sky. Blue-white Sirius (pronounced SEER ee us) marks the top of the Upside Down Y asterism that defines Canis Major.


The Winter Triangle

8) Procyon is the brightest star in Canis Minor the Little Dog. Procyon (pronounced PRO see ahn) is a yellow-white subgiant star. Procyon, along with Sirius and the red supergiant Betelgeuse in Orion, mark the corners of the Winter Triangle asterism.



9) Pollux is the brightest star in Gemini the Twins. Pollux (pronounced PAH lucks) is an orange giant star with a planet three times the size of Jupiter orbiting it. Currently, if you draw a line between Aldebaran and the copper-colored planet Mars (which is slightly brighter than Aldebaran) and continue past Mars, you will come to a pair of bright stars. The southernmost of the pair is Pollux, slightly brighter than the other star, Castor. Castor and Pollux are the names of the Gemini twins.

So there you have it, the best and the brightest that the winter sky has to offer. Collect all six.