In addition to the traditional navy blue Tee's and Sweats, this new delivery includes a reverse variation. High quality Tee's and Sweats in white fabric with navy blue logo are available. The shirts bear the club logo on the front, as before, and the caps are solid fabric (as opposed to the mesh), in navy blue and with the white logo up front.
Prices are reasonable as in year's past and include a small donation to the club fund. Sizes and
styles are limited and several of the white shirts have already been sold. If members wish to
reserve an item, so as to assure delivery, contact Bob Tillotson at 773-8106 before the meeting.
Support you club. Buy a shirt and cap and display the club colors at the meetings and events.
1) May 10, Antelope Island at White Rock Bay. Members can arrive early, enjoy a tale gate party and show visitors the sun through solar filters.
2) May 16, Green Acres Elementary School. The school is located at 640 East 1900 North in
Ogden. The school is expecting in excess of 300 students and parents. Members are needed
with their telescopes. The school is making a cash donation to the club. Assemble telescopes by
8:30 p.m.
3) May 17, Harmon's Shopping Center at 2nd Street and Washington Blvd. This is really taking astronomy to the people. Though the conditions will be likely require sunglasses, even after sunset, there should be hundreds of curious visitors to look through the 'scopes. Assemble by 8:30 p.m.
If there are questions concerning any of these events, they will be answered at the May 8
meeting, or, call Steve, Elgie, Bob or Doug, (you know their numbers...).
OGDEN ASTRONOMICAL SOCIETY
April 10, 1997
The regular meeting opened at 7:30 p.m., President Steve Peterson directing.
The Barnes and Noble star party washed out twice. No plans to reschedule.
Because of poor weather but high interest, another Antelope Island event is scheduled for April 26. Follow the signs to White Rock Bay.
John needs help for this quarter's final Wed. Night star party at W.S.U. Arrive by 8:30 p.m.
Elgie Mills and Jim Seargeant showed the latest results of their CCD and film photography. Some incredible images of the comet were shown.
Planetarium assistant Jarett Bartholomew ran "Voyage to the Planets".
Meeting adjourned at 8:40 p.m. August 29, 1997"
by Mike Reinsch
"I'll still be back"
For me, the Moon has been something that I
occasionally take a glance at when I'm not
observing galaxies and nebulas. However, I
would like to share some very interesting facts
about the Moon with you.
One question that pops up in relation to the Moon is: Why are there two high tides almost every day? Of course, we all know it has to do with gravity, but shouldn't there be only one high tide a day since the Earth rotates, in relation to the Moon, only once a day? The answer lies not only with gravity, but also in the amount of gravitational difference over the whole Earth.
The distance from the center of the Moon to the closest side of the Earth is 234,000 miles. To get the distance from the center of the Moon to the farthest side of the Earth we must add the thickness of the Earth (8,000 miles) to the first figure. We then get a distance of 242,000 miles. If we set the distance from the center of the Moon to the nearest side of the Earth at 1.0, then the distance to the farthest side would be 1.034. As the distance increases from 1.0 to 1.034, the gravitational force decreases from 1.0 to 1/1.034 squared, or .93.
Therefore, there is a 7.0% difference in the amount of gravitational force exerted by the Moon on the two sides of the Earth. From this we can reason that the side of the Earth facing the Moon will have the most amount of gravitational force exerted on it causing a tidal bulge, and the side of the Earth not facing the Moon will have the least amount of gravitational force exerted on it causing another tidal bulge. We therefore have two tidal bulges every day, even though the Earth only rotates once per day.
But, remember I said that there were two tidal bulges almost every day. If the Moon were motionless, then the tidal bulges would be in the same place every day. But, the tidal bulges are not in the same place every day. This is because the Moon itself is orbiting the Earth in the same direction that the Earth rotates. By the time some point on the Earth has passed through one tidal bulge and is approaching the second, the second bulge has moved onward so that the Earth must rotate an additional half hour to catch up with it. This means that every day the tidal bulges arrive nearly one hour later than the previous day.
Another question concerning the Moon is: Why does one side of the Moon always face the Earth? (I have to say here that the answer to this has always fascinated me.)
As the tidal bulge moves along the surface of the Earth, it scrapes along the bottom of the sea floor and the energy of the Earth's rotation is dissipated. However, the energy of the Earth's rotation is so huge that this dissipation amounts to only a tiny fraction of the total amount. Still it is enough to be continually slowing the Earth's rotation by one second every hundred thousand years. This isn't much by human scale, but over the four and a half billion years that the Earth has existed, the day has lengthened by nearly fourteen hours. Just think, when the Earth was first born out of gas and dust, it must have been rotating with a period of only ten hours or possibly less.
As the Earth slows down it loses angular momentum, but the Earth cannot lose this angular momentum as dissipated heat. The angular momentum must be retained elsewhere in the Moon-Earth system. What the Earth loses the Moon must retain, and it does this by receding from the Earth. The angular momentum of an object depends not only on the period of rotation, but also upon the distance an object is from the center about which it turns. (An analogy would be if you take a baseball and somehow connect a string to it, then start whirling it around you about a foot away. Now try doing the same thing only with the ball eight feet away from you, there's a difference. As the ball moves away from you, it gains angular momentum.)
The effect of the tides, then, is to slow the rotation of the Earth and to increase the distance of the Moon.
There is a limit to how much the Earth's rotation will be slowed. Eventually, the Earth will rotate about its axis so slowly that one side will always face the Moon as the Moon circles in its orbit about the Earth. When that happens, the tidal bulges will be "frozen" in place and will no longer travel about the Earth. The length of the Earth day will be about fifty times as long as the present day.
Since the mass of the Earth is eighty-one times that of the Moon, you might think that the tidal effect of Earth-on-Moon would be eighty-one times that of the Moon-on-Earth. However, the Moon is a smaller body than the Earth so there's a smaller amount of gravitational difference over its width. If the effect of Moon-on-Earth is considered to be 1.0, then the effect of Earth-on-Moon is 32.5.
With the Moon effected 32.5 times as much as the Earth is, there has been ample time in the history of the solar system for the Moon to dissipate its rotational energy to the point where its tidal bulge is frozen, and where the Moon faces only one side to the Earth. This is actually the case.
But, let's not stop with the Moon. We can reason from this that any satellite which receives a
greater tidal effect than that of the Earth-on-Moon would also have one side permanently facing
its primary. As a matter of fact, there are six other satellites in the solar system that are
Moon-sized or larger, and each is gravitationally attached to a primary much more massive than
the Earth.
Photograph courtesy of the Hansen Planetarium
Cite
(1) Asimov on Astronomy, Isaac Asimov, 1979
If you have any comments or have noted any errors, please let me know. My e-mail is
mike@azaccess.com.
By Jim McCormick
Having gotten off to poor start with last month's column, I vow to do better in May. For those of you who may have gone out looking for an occultation of Aldeberan on April 7, my apologies---that event took place three days later. Although it was cold and windy that night, I observed the occultation with binoculars through my living room window. The disappearance of the star was so abrupt, you could almost hear a "clap". The other problem concerned the observation of periodic comet Wild 2. Even though the positional information I provided was ok, I overlooked the location of the Moon on the evening in question. It was a less than eight degrees away from the comet. I did try to glimpse Wild 2, but the Moon, 64 percent illuminated, was too close and too bright. But, we can try again this month.
Pluto
Most amateur astronomers have observed at least seven of the nine "recognized" planets. Earth is a freebie and one of the seven. It is astonishing that there are a few of us who have never seen Mercury. Many more have not yet seen Pluto. This Summer we can try to remedy this disturbing situation. Since Mercury orbits the Sun every 73 days, it makes several appearances in the morning and evening sky each year. On some of these visits, the inner-most planet, due to the inclination of its orbit, stays pretty close to the horizon and because of its swift motion, can only be seen for a week or so. Over the next few months Mercury's favorable observation opportunities will be reported.
Pluto is another story. It is faint and difficult to recognize against a background of faint stars. Right now Pluto is inside the orbit of Neptune. Being closer the Sun than usual (this is the first time Pluto has been observed closer to the Sun than Neptune since Tombaugh discovered it in 1930) it's about as bright as we will ever see it, but still only about magnitude 13.6. Although I have found Pluto with a 10" reflector, I have been told Pluto can be seen with an 8" SCT (which I have tried to do, without success). This year
Pluto reaches opposition on June 17th and May, June and July offer goods opportunities to see
this smaller-than-the-Moon planet. Incidently, on June 17th Pluto will be 242 light minutes
away.
Sky & Telescope and Astronomy magazines offer finder charts for Pluto each year and they can be used to help you locate it. From personal experience I have found such charts difficult to use unless Pluto is located close to a star that is relatively easy to identify.
Consequently, I will try to guide you to
Pluto only on those occasions when Pluto
is most conveniently positioned. For
example, try finding Pluto on the night of
June 2-3 when the Planet can be found by
star-hopping from 5.6 magnitude 18
Scorpius. The following figure shows the
location of 18 Sco. Use this chart and the
following instructions to find Pluto.
Figure 1
Begin by locating 18 Scorpius (R.A. 16:15:37, Dec. -8° 21'). For most sky conditions 18 Sco. will not be a naked-eye star so a little help is in order. If you are using an aligned telescope with setting circles, look for the brightest star in the field when pointed at the coordinates given above. For Telrad users, center the Telrad bulls eye on Zeta () Oph (a naked eye star---check your star charts). Move the telescope until Zeta lies just outside the outer circle on the bulls eye at about 7:30 0'clock. You should now be centered on the 4.6 mag star Upsilon ()Oph. You will find 18 Sco. exactly 3° west of Upsilon Oph. If you ever wondered about the circles on the Telrad bulls eye, the inner circle has a radius of 15' (1/4 degree). The middle one has a radius of one degree and the outer a radius of two degrees.
Figure 2
Once 18 Sco. is located, the rest is easy. Using moderate
magnification (a field 30'-40' in diameter, about the width
of the full Moon), center 18 Sco. in the field then move the
telescope to the east until 18 Sco. is well to side of the
field. You will see two relatively bright (magnitudes 8.5
and 9.5) stars enter the field from the opposite side. Now
continue to move in the same direction until two additional
stars move into the field forming a crude trapezoid with
the first two stars. This latter pair of stars is the key to
unlocking Pluto's position. On June 3, a little after
midnight (MDT) Pluto can be found almost exactly 1/3 of
the way from the southernmost of the pair to the
northernmost.
It is recommended that you practice locating locating 18
Sco and the "trapezoid" a few times prior to June 2nd. Be
sure your telescope has enough aperture to observe stars
fainter than 13th magnitude. For a shortcut to finding the trapezoid, it is centered on R.A. 16:17
South 8°16.5'. And good luck! Maybe you can add Pluto to your list of observed planets.
Second Shot at p/Wild 2
OK---Let's try periodic comet Wild 2 again.
This time the Moon will be far away and below
the horizon. The night will be May 26, the time
10 PM MDT. The jumping off point is an easy
find, Regulus or Alpha Leonis. See the chart
below. 2.7° from Regulus at PA= 48.2° can be
found the 5.4 magnitude star 37 Leonis. Just
21' further in the direction PA=19.8° you will
hopefully find Comet Wild 2 [For those
unfamiliar with position angles (PA), these
angles are measured in a counter clockwise
direction from the north. For example, if object
B is southwest of object A, it would have a PA
of 225 relative to object A]. It is difficult to say
what its magnitude will be at that time, but a
guess would ~11. Give it a try.
Figure 3
Antelope Island Star Party May 10
The monthly public star party on Antelope Island, Saturday, May 10 will again feature Comet Hale-Bopp. Close by H-B on that evening will be four other objects worthy of our efforts to observe them. Within a square 20 degrees on a side can be found Comet Hale-Bopp, Aldeberan, Venus, Comet Encke and the Pleiades.
Figure 4
Comet Encke reaches
perihelion on May 23 and
may be as bright as 7th
magnitude. If the sky is
dark enough at 9:00 P.M.
and if Buffalo Point does
not obstruct the view, we
may be able to see Encke
as well as Comet
Hale-Bopp. Venus will be
just 3.6° above the horizon
and has the lowest altitude
of this set of objects. The
Sun sets at 8:34 P.M. and Civil Twilight begins at 9:06 P.M. The figure above shows the setting
times and the altitudes and azimuths of each object for 9:00 P.M.
*****
Editor's Note:
The Editor regrets that, due to technical difficulties, the graphic charts in Jim's article failed to
reproduce clearly. For clear and complete reproductions of figures 1through 4, please contact
Jim McCormick directly.
Figure 1. This CCD photograph of Comet Hale-Bopp was taken on October 11, 1996 with Elgie's ST-5 camera. At that time the comet was 3.026 A.U. from Earth and 2.755 A.U. from the sun and in the constellation Ophiuchus.
Even at this time a gas jet from the
nucleus was visible at about the 4
o'clock position. These eventually
began to form the now famous Bow
Waves, (see below).
Figure 2. Another CCD image of comet Hale-Bopp taken by Elgie, this time on April 3, 1997. At this time, the comet was two days past perihelion or .915 A.U. from the sun and 1.376 A.U. from Earth. The Bow Waves, created by the out gassing from the nucleus and its rotation are clearly visible in this f/4 exposure.
Figure 3. This image of the famous
"Whirlpool Galaxy", M 51, was
among the first pictures Elgie took.
One of his favorite objects in the
camera or through the eyepiece, he
has probably taken more exposures
of this object than any other,
(except Hale-Bopp).
The galaxy contains many billions
of stars. It's light has traveled
about 35 million light years thus
we observe this beautiful object as
it "was" 35 million years ago. M
51 is in the constellation Canes
Venatici.
Text is by the Editor