Posts Made By: Curt Renz

At least one media source is reporting that the next penumbral lunar eclipse after this month’s event will occur in 2053. That’s only true for “Total” Penumbral Lunar Eclipses. Actually, penumbrals occur fairly frequently. There will be three in 2009, but none a TPLE. And every total or partial lunar eclipse is preceded and followed by penumbral stages.

Despite the fact that after this month’s TPLE the next will be in 2053, we are nevertheless living in a rich period for such events. There exists a cycle of roughly 565 years for many factors involving both solar and lunar eclipses. There are no TPLE’s during the periods 1666-1805 and 2223-2428. There were 10 such events during the period 1900-1999. During this century they will occur in 2006, 2053, 2070, 2082 and 2099. We’re also living in a rich period for total lunar eclipses, including groupings of four of them known as tetrads as we had in 2003-4 and will again in 2014-5.

A penumbral eclipse is not a spectacular event. For many of them the slight darkening of the Full Moon goes unnoticed. However, when the Moon’s limb gets within about 30% of a lunar diameter from the Earth’s umbra, a dimming on the side near the umbra can be perceived. That will definitely be the case this month when the Moon misses the umbra by about 7% of a lunar diameter.

GLOSSARY:

Umbra - The Earth’s umbra is the dark portion of its shadow. An observer standing in the umbra would not be able to see any portion of the surface of the Sun since it would be completely obscured by the Earth.

Penumbra – The Earth’s penumbra is the outer gray fringe shadow surrounding the umbra. An observer standing in the penumbra would see the Sun being partially obscured by the Earth.

If you truly want the question "fully" answered, then read what I have written below. It describes the factors that create differences between the lengths of the true solar day and the mean solar day leading to apparent anomalies in the times of sunrise and sunset. Sidereal time plays no role in my explanation.

There are two factors. The primary one is simply an effect of spherical geometry on the apparent celestial sphere surrounding the earth. The secondary factor is the eccentricity of the Earth’s orbit and the resultant changes in its angular velocity around the Sun. Even if the Earth’s orbit were not eccentric, the primary effect due to spherical geometry would still be quite noticeable regarding sunset and sunrise times and the time of the Sun’s meridian crossing (true noon). The earliest sunset would still occur a number of days before the winter solstice. The latest sunrise would still occur days after the winter solstice. The earliest sunrise would still occur before the summer solstice. The latest sunset would still occur after the summer solstice.

The combination of the two factors results in what is known as the Equation of Time. That is the difference between sundial time and that of our standardized clocks after a constant adjustment for longitude. In February sundials are 14 minutes slow. In May they are 4 minutes fast. In July they are 7 minutes slow. In November they are 16 minutes fast. Again, the main reason is spherical geometry. However, if the Earth’s orbit were not eccentric, the absolute values of those four numbers of minutes would be identical. The Equation of Time equally alters the times of sunrise and sunset for observers at all locations on Earth, including those on the equator.

A true solar day is the length of time between local meridian crossings by the Sun (true noon to true noon.) The Equation of Time is actually the sum of daily variations from 24 hours (a mean solar day) in the length of a true solar day: 23:59:39 hr to 24:00:30 hr as measured by mean time.

If we used sundial time, the apparent anomalies of sunrise, sunset and meridian crossing times would disappear, because we would be observing the true solar day. With standard time we utilize the mean solar day. Of course, in either case the graphs of sunset and sunrise times would loosely approximate sine waves. At the extremes of the graphed times, the daily changes of those times are minimal. Midway between the dates of extreme sunrises or sunsets, the daily changes would reach their maximums. But when reading a sundial, the daily change in both the sunrise time and the sunset time would be virtually identical in absolute value between any particular pairs of consecutive dates. Thus if your relative had been using a sundial, he would not have posed his question.

The geometric effect can be imagined this way. At the solstices the Sun reaches its maximum declinations on the celestial sphere. The meridians of right ascension are more closely packed than at the celestial equator. The Sun in its apparent motion through the equatorial coordinate system gains more each day in right ascension than it does around the equinoxes. Also, at the equinoxes the Sun would appear to be moving in a slant relative to the parallels of declination. Our standardized clocks are based on a fictitious mean Sun that advances in right ascension at a uniform rate. As a result of the spherical geometry, the true Sun’s right ascension is ahead of the mean Sun’s right ascension in February and July; it is behind in May and November.

The variable angular speed of the Earth in its eccentric orbit modifies the previously described geometric effect. The fact that the Earth is moving faster (and gaining more in right ascension) in January than July results in greater extremes in the Equation of Time in November and February than in May and July.

The basic sunrise or sunset times as measured by a sundial obviously go through one complete wave cycle each year. The orbital effect on the Equation of Time also goes through one wave cycle each year. The geometric effect goes through two wave cycles each year. When the three waves are combined the result is somewhat complex. If that is bothersome, then throw away your watch and use a sundial.

P.S. In the interest of relative brevity, I've left out any discussion of how the Equation of Time and the times of sunrise and sunset change over the centuries due to the precession of the equinoxes and changes in the Earth's orbital eccentricity and axial tilt. BTW, the current mean value of that tilt is 23.438° (not 22.5°)and will continue declining until it reaches a minimum of 22.6° in 10,000 years.

I agree: 8 major planets. In particular, the inclusion of Pluto’s satellite Charon in the list of 12 major planets seems ludicrous. Many other planetary satellites, including the Moon, are far larger. The argument seems to be that the Pluto-Charon barycenter lies above Pluto's surface. For the Moon and the other large satellites the barycenter lies within the main planet. Future students memorizing the list of planets will have a hard time understanding the difference that caused the Moon to be left out.

I would think that the shape of a planetary satellite's orbit relative to the Sun would be a greater consideration than barycenter. As with most non-retrograde satellites, Charon describes an epitrochoid relative to the Sun. In particular this is the type of epitrochoid that is alternately convex and concave with points of inflection (wavelike), e.g. Jupiter’s Ganymede and Callisto. Other satellites describe epitrochoids that cross over themselves (loop-the-loop), e.g. Jupiter’s Io and Europa.

The Moon is unique in that its orbit is perpetually concave toward the Sun. We think of the Earth as orbiting the Sun while being perturbed by the Moon. Normally we consider the Moon as orbiting the Earth while being perturbed by the Sun. But more so than other satellites, the Moon could also be thought of as orbiting the Sun while being perturbed by the Earth.

I would refer to the Moon as a planet before I would do so for Charon. Geophysically, I have always thought of the Moon as the first extraterrestrial planet visited by astronauts. Historically, there were seven planets (wandering stars): Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. Although in most contexts the term referred only to the latter five, as the Sun and Moon were considered to be of elevated importance to ancient naked-eye observers.

In truth, I am not calling for the Moon and other large planetary satellites to be included in the list of major planets. I’m just asking that Charon be crossed off the list. It makes those seven committee members who agreed to it look silly. They almost have me wondering if they intended to draw up a list so ridiculous that it might make others stop insisting on the inclusion of Pluto.

I totally agree with 8. I had more to say about Charon within another post in this thread.

Regarding Ceres, I believe that nineteenth century astronomers wisely demoted it from initial planetary status after they discovered some of its brethren. Their small sizes, and highly eccentric and inclined orbits indicated the need for defining a new class. Those earlier astronomers felt a classification called asteroids would be more helpful in guiding the research of those seeking to understand the dynamics and origin of the solar system. For the same reason, I believe it would make more sense to place all KBO’s (including Pluto) in a class of their own.

Here is a link to a petition for those opposing the committee’s suggested definition of planet. It is supposedly for those preferring 9 planets rather than 12, but those favoring 8 can add a comment as I did. http://www.petitiononline.com/9planets/petition.html

Q1) What does one call a horse designed by a committee?
A1) A camel

Q2) What does one call a set of planets chosen by a committee?
A2) Wanderingstarsasteroidsgasgiantssatelliteskuiperbeltobjects

The Trans-Neptunian Object preliminarily assigned the label 2003 UB313 has now been given the permanent designation Groucho. It was immediately informed that it is being considered for membership in the elite club of major planets. Groucho quickly declined, quipping, “I don’t care to belong to any club that will have me as a member.”

The southern graze path of the second brightest Pleiades star, Atlas, will pass through Los Angeles. It will enter land at Santa Monica then sweep through Beverly Hills, Burbank, Verdugo City and La Crescenta. Listed below are the graze points I’ve calculated for several meridians across the metropolitan area.

Atlas -- Mag 3.6 -- RA 03:49:35.0 -- Dec N 24°04’36”
Lon W ----- Lat N ------ PDT --- Zenith Angle
118.5° -- 34.00988° -- 22:37:31 -- 217.9°
118.4° -- 34.08551° -- 22:37:38 -- 217.9°
118.3° -- 34.16123° -- 22:37:46 -- 217.8°
118.2° -- 34.23704° -- 22:37:54 -- 217.7°

Here's a formula for calculating the graze path in the LA area only:

W = Longitude - 118
Latitude = 34.38895 - 0.76048 W + 0.0047 W²

Really fine scene, Robert. Thanks for sharing. Glad you liked the Oboe. Sorry that the lady was not yet in the mood. :S

It looks as though skies will be overcast tonight here in Chicagoland. The weather gods have been unkind to us throughout this Pleiades occultation series. Hopefully, weather conditions will be more agreeable in your area. If they are, your photos of the occultation would be welcome additions to this thread. In any event, there will be another opportunity to witness a Pleiades occultation during the evening of December 3. I'll provide details in a new thread well before the event occurs.

Yes indeed, it would happen twice every Saturnian year at its equinoxes, since the rings are right over Saturn's equator, as Maurice noted. However, since Saturn's orbit is eccentric, the separations between equinoxes alternate between 15 years 9 months and 13 years nine months. The next equinox for Saturn will occur on 2009 AUG 10. The Earth will pass through the plane of Saturn's rings on 2009 SEP 04. The following equivalent dates are 2025 MAY 06 and 2025 MAR 23. It’s estimated that the rings will seem to disappear for earthbound observers for about 25 days in 2009 and 44 days in 2025.

Since the rings are so thin, an observer on the equator of Saturn at the equinox would only see a stripe across the Sun. The observer would be in the penumbra of the ring particles, not the umbra. So in effect it would be a partial eclipse with most of the sunlight coming through. However, for an observer on the inner edge of the ring there would be substantial blocking of sunlight since he would be in the umbra of individual or groups of particles.

Since the rings are made of countless small satellites, they would shine in the nighttime sky for an observer on the "surface" of Saturn just as the Moon does for us. However, they will often be eclipsed by Saturn's shadow in the manner of a lunar eclipse.