OBSERVING THE MOON Last updated: 23 April 2006

OBSERVING GUIDE: OUR ROMANCE WITH THE MOON

INTRODUCTION

With all of the latest sophistication in astronomy.....the Hubble Space Telescope, deep space probes that land on potato-shaped asteroids....telescopes that GO TO objects and don't even need an observer to point them...

.....we tend to overlook the fundamental excitement of astronomy and why we are compelled toward it in the first place: DISCOVERY. It is the "unknown" that first lured us to astronomy, the challenge to use our minds and imaginations to unlock mysteries that we simply did not know answers to....and, for the most part, never will. It is still human nature to "explore and discover."

And "discovery" is what led us to the moon. We've been there - and as the television commercial hawking cheese says: "we found out that the moon was NOT made of cheese....and we haven't been back since."

Nonetheless, the moon remains a wonderful object to explore, even in this high tech era of computerize telescopes and satellites filling our skies. It's so close, and yet we tend to ignore it! I certainly do not know EVERYTHING there is to know about the moon....do you? Not one of us can randomly aim a telescope at high power toward the lunar disk and call out the names of even a fraction of the craters seen, their history, their approximate ages.....so WHY are we not looking at this grand neighbor more?

The moon presents as a wealth of exciting craters, mountains, valleys, plains and even perhaps darkened volcanic cones, all of these clearly visible in our modest telescopes; not rarely there are lunar eclipses where the Earth's shadow paints a reddish hue across the lunar face. Also the moon creeps eastward each night, covering up stars in its path making for an exciting night of celestial "hide-and-seek" of distant stars.

So let's explore some wonderful some wonderful facts and exciting events about our moon so that you can explore the lunar surface and lunar events with your ETX and LX 90 telescopes tonight! We CAN bring back our romance with the moon! In case it has been longer than we care to admit, or simply the fact that we never bothered to learn much about our companion, Part II of this ETX / LX 90 LUNAR OBSERVING GUIDE contains a few textbook "basics."

One study that is fun for the entire family or "ETx family" is the observations of "lunar conjunctions" and "occultations." Not only are these visually-exciting events (see "Welcome to My Neighborhood" photo), but in the long run an amateur astronomer and his or her modest telescope can make a substantial contribution to astronomical science as well.

One thing is for sure....you most definitely will not need the GO TO for this one!

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PART 1 - THE MOON'S PATH: Conjunctions & Occultations

Daily Motion of the Moon -

In addition to orbiting around the Earth with respect to the background stars in a period of about 27 days 8 hours, the moon is part of the Earth-Moon duo orbiting the sun almost 1/13th of a year; simply put, this results in its revolution around with respect to the sun at a longer 29 days 13h, its synodic period, that on which the modern calendar is based. It is the difference between the two that also allows for the varying lunar phases - from new to quarter, from quarter to gibbous, from gibbous to full, from full to waning gibbous, from waning gibbous to three-quarter, and from three-quarter back to new - all in the course of about what we call a "month." Aspects of the lunar phases and observing are discussed in Section II of this Guide.

The moon, if observed in your telescope against the background stars at night, moves about 13º eastward in the course of one Earth day. As we see the sun from Earth on the other hand, its motion eastward is only about 1° eastward in the same period. The resulting net 12º eastward motion of the moon each day delays the rising of the moon on subsequent nights by about 50 minutes. If the moon transits overhead tonight at 10 p.m. local time, tomorrow night the transit will occur at 10:50 p.m. local time. Thus, if bright light of the full moon (transiting overhead at midnight local time) is blocking your viewing of faint constellations and stars tonight, remember that in only six days the moon will not even rise in the east until slightly after 11:00 p.m.! All the hours of darkness prior to that time can be spent stargazing in a moonless sky!

In addition to this long-term effect of daily motion, we can experience first hand the motion of the moon in an even shorter period. The 12° daily eastward motion of the moon can thought of in smaller terms: each hour this translates in the moon moving its own diameter - one half degree - eastward. (See Figure 1) What does this mean to all of us with small and large telescopes? Using very low magnification - even binoculars - it is possible to watch during that one hour the moon creep slowly eastward against the far-distant stars, the stars appearing motionless. Indeed, there are many, many time in the course of a 24-hour day that at some locations throughout the world that the moon's hourly path actually takes its "disk", or globe, to pass in front of the stars, a phenomenon known to astronomers as lunar occultations. A bit later, I will discuss the occultation phenomenon events that you can observe through your ETX 90, ETX 125 and LX 90 telescopes!

Do not confuse the "occultation" with a more common event: "the lunar conjunction," in which the moon appears very close in the sky to a bright star or planet. The "Welcome to my Neighborhood" photograph shows a beautiful lunar occultation with the planet Venus in the late 1970's. Conjunctions are fairly common events, particularly when Venus is high in the evening western sky during "early" crescent phases (See Figure 6) of the moon, and when Venus is a morning object in the eastern sky. All of the bright planets are frequently seen "in conjunction with" the moon.

An "occultation" in astronomy is an event in which one celestial object covers another.....indeed, even a solar eclipse is an occultation in itself, as the moon simply covers the sun. Although not rare, they can be spectacular if a bright star or planet is involved. As well, occultations are of valuable scientific importance. Astronomers utilizing very high power in large telescopes can monitor the exact time (to 1/10th second by eye and 1/100th second using electronic photoelectric devices) at which the star "blinks out" as it is covered by the dark east edge (or "limb") of the moon. This allows mathematical computation of the exact location and distance of the moon over a long period of time, a calculation that has revealed that the moon is very slowly - very slowly - moving ever-so-farther from the Earth, at a rate of about 3--4 cm/yr.

The tidal bulges on the Earth main toward the middle of the oceans), raised by the Moon's gravity, are rotated forward (ahead of) the Earth-Moon line by the Earth's rotation since it is faster than the Moon's actual motion in its orbit. The gravity from these leading and trailing bulges impels the Moon forward along the direction of its motion in orbit and the force transfers momentum from the rotating Earth to the revolving Moon, simultaneously dragging the Earth and accelerating the Moon.

This ultimately causes the moon to recede from the earth as it angular momentum increases.

For the purpose of this Guide the daily motion of the moon is very simple to keep up with and very interesting as well, since it allows for the moon to actually be observed through your ETX or LX 90 telescope moving against the stars in the sky during an evening's observing. Understanding the not-so-simple movement of our natural satellite also can help you appreciate - and explain to the family - how the moon managed to "....creep away from Venus since last night" and show up far east of where it was only 24 hours ago, and why that thin crescent is destined to become a bright, full moon rising in the east in less than only two weeks!

The Ecliptic and the Motion of the Moon -

If you watch the motion of the moon, the sun and even the planets from day to day, month to month and ultimately from one year to the next, you will soon realize that all these solar system objects move in a well-defined, narrow and repeating path across the sky.

Many solar system objects appear to move very fast against the stars, like the sun and moon. Part of this apparent rapid motion is due to the short distance of the object to the Earth, but a great deal of it is due to the motions of the Earth itself (both from ROTATION in a 24-hour day, and its REVOLUTION around the sun for the 365-day "year"). Still other objects - Jupiter, and Saturn for example - appear to move very slowly because of their great distance from our "observatory": Earth.

So in addition to our moon, all of the objects in the solar system move within a very narrow "belt" of the sky that astronomers today call the ecliptic; even the earliest star gazers of yesteryear realized the motions of "the wanderers" (the seven naked-eye "planetes" including: Mercury, Venus, the Moon, Mars, Jupiter, Saturn and the Moon) in this band and entitled it the zodiac (Greek, for "Zone of the Animals)." Through the twelve constellations within the zodiac the wandering planets, sun and moon all travel, and give rise to the centuries-old attention to the pseudoscience of astrology.

The twelve zodiacal constellations and their common names are (beginning with the First, Aries, and progressive eastward): ARIES (the Ram); TAURUS (the bull); GEMINI (twins); CANCER (the Crab); LEO (a lion); VIRGO (young virgin); LIBRA (balance - the only "non-animal of the group!!); SCORPIUS (the Scorpion); SAGITTARIUS (an Archer); CAPRICORNUS (a "sea Goat"); AQUARIUS (the Water-Bearer); and PISCES (the Fish).

Each of these constellations of the zodiac (a "sign" in astrology) holds special meaning to those born "under the sign," even to astrologers of the twenty-first century.If the moon, as well as all solar system objects, appear to move only within this narrow band of sky, does that mean that occultations of stars take place every month as the moon orbits the Earth??

The answer is "yes".....and, "no."

YES: Occultations of stars and other objects by the moon occur every night as seen from Earth. The moon is moving so rapidly eastward across our sky that not a night goes by at someplace on the planet that multiple occultations do not take place.

NO: The same objects are NOT always occulted each month as they were the previous month, nor are they occulted by the moon necessarily in the course of one year. The actual reason is complex, but the moon, relative to the Earth, sun and stars returns to exactly the same position as we see it only every 18.6 years.

All solar system objects, the moon included, follow a pathway through the ecliptic, an imaginary plane intersecting the sun's equator and containing all of the orbits of the planets and of their natural satellites. Imagine the sun as a huge semicircle, a globe cut in half and laying across a large table. On the table marbles roll around the semicircle like planets around the sun.

While all planets move in individual orbits of their own, their apparent motion is a result of these orbits, and the way we see them moving from Earth.

Part of this combined motion can result in the apparent motion of any celestial object varying as much as a 13° "up and down" (north and south) from the true plane of the ecliptic (the flat "tabletop"). So - although the moon passes close to the same stars at least once monthly - its proximity and ability to intersect and occult those stars will vary, albeit predictably by astronomical calculations.

Simply stated, if the bright star Spica is occulted by the moon tonight, an occultation of the same circumstances (see Figure 1) CAN occur from your same exact location on earth again in about 18.6 years.

Occultations for the Casual Observer - A lunar occultation is a sudden and spectacular event; many times, even with binoculars and unknowing of an upcoming event, an observer might spot a bright star "flash away" as the dark edge of the moon suddenly covers the star (Figure 1). However, the nature of the moon's motion and the seemingly fixed positions of stars within the ecliptic allows astronomers to predict and publish such occurrences years in advance. Note that occultations of BRIGHT stars, any of the "superior" planets (those more distant than Earth from the sun), ASTEROIDS and even deep sky (i.e, the Pleiades star cluster) are all published in Sky and Telescope magazine far ahead of time to alert you to these upcoming spectacular events.

Note that a star - because of its great distance, even those closest to us - "blinks" out behind the limb, or edge of the moon. A PLANET, on the other had, does not "blink out" because it is NOT a point source; its disk merely gradually diminishes in brightness (see Figures 4 & 5) as it progressively disappears behind the lunar limb (edge). Likewise, "multiple stars" - even those that cannot be separated into two or more components in you telescope -also have a double fading, with one component disappearing (the light suddenly diminishes) and suddenly the other, causing both stars (or more) to finally "blink out." Actually, several new double stars - and "double asteroids" - have been DISCOVERED using the occultation "double-blink" method!

Star "blinking" is a result of two factors:

1) Because even the closest star is farther than 12 trillion miles distant, their apparent size, or angular size, is equal to that of a point of light, showing no diameter which can be slowly covered up by the edge of the moon; the largest stars may be over a million miles in diameter, yet their distances still show them to us on Earth as a tiny point of light.

2) The moon has no air surrounding its surface like the Earth's atmospheric "blanket." Hence, there is no gradual diminishing of light as would be expected as the star creeps deeper and deeper into an atmosphere. Such a dimming of starlight by the atmosphere is called "atmospheric extinction" and is particularly noticeable on Earth as bright stars grow dimmer and dimmer the closer they get to the western horizon during setting.

Occultations are much easier to spot and record during waxing phases (Figure 1), those lunar phases between new moon through the next two weeks to full moon. The sunlight which illuminates the moon during such times is striking the western side of the moon, leaving the eastern or approaching side darkened. Even faint stars stand out in stark contrast to the dark limb of the moon.

Many times, particular close to either waxing or waning crescent moon, a bright reddish reflection of sunlight off the shiny atmosphere of Earth can be seen distinctly on the "dark side" of the moon. In addition to being a striking sight, it allows you to define the moon's edge and see the brightest craters even in full darkness. This gentle glow on the lunar surface is known as "earthshine," and is exactly the same phenomenon that Earthlings realize when walking about in the midst of brightened sky and ground of full moonlight.

Stars that normally would be visible without the bright full moon, are blocked by the scattering in our air of bright reflected sunlight from the moon. Because the moon, has no air, the faintest of stars can be clearly seen during "full Earth" (the Earth's phases as seen from the moon are exactly opposite those exhibited by the moon on the same given night!

As previously mentioned, occultations of stars and planets by the moon during waning phases - or those that occur within the two weeks following a full moon - take place with the object disappearing on the bright limb of the moon. This means that the bright lunar edge greatly impairs your ability to view the star and thus its sudden disappearance. Indeed, during the waning phases of the moon, it is more desirable to watch the reappearance of a bright star as it suddenly "blinks" into view out from BEHIND the moon on the darkened eastern limb.

Of course, your might wish to view BOTH occurrences - the disappearance and the subsequent reappearance of the same star. Because the moon moves its own diameter in one hour, a star that disappears directly in the "middle" of the lunar disk (as shown in Figure 1), will re-emerge from behind the opposite edge (limb) in about 50 minutes to one hour. Those that are occulted nearer the lunar poles will reappear in a much shorter interval that those near the lunar equator. The length of time, thus is dictated by what point relative to the moon's equator the star disappears, the farther from the moon's midsection (toward the lunar poles), the shorter time until it reappears on the opposite limb.

REMEMBER!! ALL stars and any object that are located or travel through the "ecliptic" are subject to occultation at one time or another!

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PART II - APPEARANCE OF THE MOON FROM PLANET EARTH

The Phases of the Moon - Although modern astronomy has been a long time coming, held back to some degree by astrological beliefs, the simple concept behind the changing faces - phases - of the moon have been understood and documented as far back as the time of Greek philosopher Anaxagoras in 430 B.C.; it was Aristotle who first chronicled the explanation in his writings so time later.

Both realized that the changing shape (illumination) that we see of the moon's surface from earth varied because of the angle from which sunlight reflected from the moon as we see it from Earth (Figure 6).

Note in this figure that the phases are like timekeepers throughout nearly a month's period. In other words, from a new moon (between the Earth and sun, therefore its illuminated face is not facing us) to the first quarter (we are seeing only a quarter of its illuminated face) is almost exactly one week. Another week passes and we see a full moon, one which we see fully illuminated by sunlight. Thus, one can expect - on average - that the moon progresses from one crescent moon (say 3 days "old") to the next successive crescent moon of three day old in a period of right about 28 days. In addition to telling the progression of weeks using the moon, one may also tell approximate local time by remembering a few basic rules for each major phase (quarter) of the moon.

Local Time by the Moon - It is easiest to start with the full moon as a means of telling time through the nightly motions of our natural satellite. Remember, to be "fully illuminated," the moon must be opposite the sun from the Earth, as shown in Figure 6 above.

Consider if the moon was on the same side of the sky as seen from Earth, or a "new moon." Obviously in that arrangement, the moon and sun would "rise" and "set" very close together on that day. In a perfect world, at the equator on Earth, that would be rising about 6 a.m., crossing nearly overhead at noon, and setting as the sun sets, about 6 p.m.

Back to the full moon ("D" in Figures 6-7) located just the opposite of the new moon relative to the sun and Earth. If opposite, then the moon on THAT night would be rising and setting opposite the sun, right?

Therefore, the full moon rises at sunset, crosses overhead (the meridian) at midnight and proceeds to set about the time the sun rises. Thus, knowing the times of sunset and sunrise on any given evening and next morning, an observer can easily reckon his or her local mean time.

Not quite as easily to visualize as the opposite principle of the full moon, time keeping with both the first quarter and third quarter moons can be just as simple. First Quarter moon is shown as position "B" and Third Quarter is in position "F" in Figure 6-7.

Knowing that the full moon keeps opposite "clock time" than the sun, the quarter moon is off, simply by six hours, since it is one-quarter of the 28-day cycle (for the purpose of this discussion) of the moon. Thus, the first quarter moon rises in the sky at noon, local time, and is approaching the meridian (highest in the sky) at sunset.

Since the third quarter moon is a quarter of the cycle past the full moon, it rises about the time of midnight, and crosses the meridian around sunrise. This is why - on a very clear, crisp morning, it is sometimes easy to spot the moon in the western sky, even after the sun is up!

The Moon's Distance, Size and Apparent Size -

Like most objects in our solar system, the moon does not move around its gravitational center (the Earth) in a perfect circle, but rather an ellipse, or slightly flattened circle. Since the moon's distance is only is only 30 times the Earth's diameter (about 8,000 miles), it is easy to recognize a slight change in the size of the moon as this ellipse sometimes brings it closer (perigee) and moves it farther away (apogee) to the Earth.

We have noted that the apparent size of the moon, some 240,000 miles distant is about 1/2° of sky. The actual size of the moon is quite small - 2,160 miles, or about on-fourth the size of mother Earth.

The moon may approach the Earth as close as 221,463 miles and - in the same month - be as distant as 252,710 miles. It is this dramatic change in proximity to Earth and its location relative to the sun which causes our ocean tides.

Why do I see the same face of the Moon, no matter what day or what time I look?......Does this mean the moon doesn't turn, or rotate? -

On the contrary, the moon rotates on the lunar axis in what may be the most peculiar, though not coincidental from a "celestial mechanics" standpoint.

But the moon DOES keep the same face, with a little "rocking" variation, called libratioin.

Interestingly, if the moon did not rotate, we would see different features (i.e., the "man in the moon") at different times - and we would be able to see the features on the far side of the moon which have never been seen from Earth!

Remarkably, the moon rotates on its axis (its "day") in almost perfectly the same period of time that it takes for it to revolve around the Earth (a lunar-Earth "year'). Thus, as it moves eastward around the Earth, it is also turning synchronously on its axis at the same rate, keeping the same face exposed to the Earth.

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The Romance of the Moon -

The moon has been the source of novels, poetry and steamy love affairs throughout history. The definitive changes in appearance, the rapid movement through the sky, the quiet suspension of its bright orb against the curtain of distant stars....are all "stuff" that makes our romance of the heavens a profound one. Indeed, even the light of the moon - from kindling love to igniting battles of war - is a reminder to us that we depend on the starry sky and all that is in it. And....more profoundly...that we are but such a small part of it all.

The bright portion of the moon is reflected sunlight, but even the full moon shines back to earth only 1/400,000 of the intense light of the sun. Indeed, the reality of a three-dimensional space is perhaps best realized during the very thin "early" and "late" crescent moons, when the bright crescent meets the dark side at the terminator where day falls instantly to night with no dusk. With only the faint glowing reddish reflection of Earth is a sphere seemingly floating silently in space on the curtain of eternity.

Astronomy and romance perhaps, do have the common thread, and nowhere is that more evident than with our bright moon. Wars have been fought by its illumination, its silent splendor have kindled the fires of love, its eclipses have cast forboding shadows of death and disease across ancient cultures.

If the true challenge of any romance is the "mystery" that surrounds it, then surely this nightly temptation of all the mysteries of the cosmos packaged neatly only a couple of hundred thousands of miles distant, affords us the ultimates in mystery....and a nightly romance as our minds travel through our telescopes to peer at mysteries locked away in a cold and silent world.

P. Clay Sherrod
Arkansas Sky Observatory
Conway / Petit Jean Mountain
Arkansas

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