Happy New Year!

The New Moon of Monday, September 29, is an important one to many people of the world. In the Hebrew Calendar, it marks Rosh Hashanah (literally, ‘head of the year’) which is the beginning of year 5769. On Tuesday night, September 30, Muslims across the world will see the first slender crescent of this lunar cycle. That will mark the end of Ramadan and the beginning of the next month, Shawwal. 1 Shawwal is ‘Eid ul Fitr, one of the greatest holidays in the Islamic calendar. This week, then, is a good time to think about the Moon, why it’s here, how it orbits, and how we have used it to measure time.

Moon and stars
Creative Commons License photo credit: joiseyshowaa

Unlike our months, Hebrew and Islamic months begin with the New Moon. Because twelve lunar months add to only 354 days, less than the 365.25 day solar year, an extra month is occasionally needed to keep the months roughly aligned with the seasons. In a 19 year cycle, years 3, 6, 8, 11, 14, 17, and 19 have the extra month. The year that is ending, 5678, is number 11 in its cycle and was a leap year.

Interestingly, the Jewish year has two ‘beginnings’. Tishrei (the month which begins now) is the first month of the civil calendar, and the month where 5678 becomes 5679. However, it is actually the seventh month of the religious calendar, which begins at Nisan (the month of Passover).

The Islamic calendar functions slightly differently. Its months begin with the first visible crescent low in the west at dusk, which is not with the actual New Moon. Keep in mind that at New Moon, the Moon is in line with the Earth and Sun, and the entire near side of the Moon has nighttime (and is therefore dark). The New Moon is visible, therefore, only if it blocks the Sun during an eclipse.

Since this New Moon occurs early Monday morning, the 29th, we expect it to be visible by Tuesday evening, the 30th. Observant Muslims, then, will continue to fast in daylight hours Monday and Tuesday. Upon seeing the Moon Tuesday night, they will know that Ramadan has become Shawwal, and they may break their fast on Wednesday.

Due to early controversy as to which years would have it, Muhammad outlawed the 13th month that kept Islamic months tied to the seasons. As a result, Ramadan (and each other month in that calendar) begins 11 days earlier each year according to our Gregorian calendar.

The Moon is Earth’s only natural satellite, orbiting our planet once every 27.3 days. However, a cycle of moon phases (say, from New Moon to the next New Moon), takes 29.54 days. This is because the Earth itself is moving during each 27.3 day Moon orbit. Since it is much easier to observe the Moon’s changing phase cycle than to observe the Sun directly, the 29.54 day phase cycle was the basis of many ancient calendars. Words for ‘moon’ and ‘month’ are related in English and are identical in many other languages. There is some evidence that our word ‘moon’ is ultimately related to an Indo-European word for ‘measure.’ Given how long we’ve measured time by the Moon, it is easy to take its presence for granted.

Released to Public: Jupiter Montage (NASA)
Creative Commons License photo credit: pingnews.com

However, our Moon is quite remarkable in several ways. Moons in our solar system are generally much smaller than the planet they orbit. Jupiter and Saturn, for example, are about 25 times bigger across than their biggest moons. Earth, though, is only 3.67 times the diameter of our Moon. Also, moons usually orbit in the same plane as their planet’s equator. Our Moon, though, orbits within about 5 degrees of Earth’s orbital plane, called the ecliptic, which is not the plane of the equator since Earth is tilted 23.5 degrees on its axis.

This leads most astronomers to believe that the Moon did not form with the Earth, but is the result of a collision with with an object roughly the size of Mars. According to this theory, the impactor (sometimes called ‘Theia’) struck a glancing blow on the Earth and was completely destroyed, and the Moon formed from the debris of Theia’s and Earth’s mantles.

This impact is what left Earth with a Moon much larger than what a planet Earth’s size would normally have, and left that Moon near Earth’s orbital plane (where the impact occured). Our relatively big moon has crucial effects not only on our tides, but also on the stability of Earth’s tilt.

Earth’s orbital tilt of about 23 and a half degrees as it goes around the Sun causes the seasons. The axis precesses, describing an aparent circle roughly every 26,000 years, but the amount of tilt (obliquity) stays nearly the same. Because the Moon acts a counterweight, the obliquity varies only between 22.1 degrees and 24.5 degrees over about 41,00 years (we are now at 23.44 degrees and decreasing). Even this orderly variation, called the Milankovitch cycle, is enough to influence our Ice Ages. Imagine the impact on Earth’s climate if there were no Moon, and the obliquity varied chaotically. This is exactly what happens at Mars, where the tiny moons Phobos and Deimos are not massive enough to influence Mars’ tilt.

Public Domain: Apollo 8 Looks at the Moon (NARA/NASA)
Creative Commons License photo credit: pingnews.com

One thing our Moon does have in common with most others is that it orbits the Earth and rotates on its own axis at the same rate. This is called ‘synchronous rotation’ and it occurs because the Moon is not exactly uniform in composition. From the time the Moon formed, the slightly heavier side was attracted to the Earth. Over time, this effect de-spun the Moon until it attained synchronous rotation. The Moon’s gravitational attraction also de-spins the Earth, although much more slowly as the Moon is less massive. As it does so, the Moon moves slightly farther from the Earth (just over 3 cm per year). The Moon is now 1.5 meters farther away that it was when Apollo astronauts went there. Don’t worry, though, by the time the Moon is far enough away to escape, the Sun will have become a red giant and swallowed both Earth and Moon anyway.

What is the shape of the Moon’s trajectory around the Sun? Perhaps not what you’d expect.

So, I encourage every one to watch for the reappearance of the Moon in the evening sky this week, even if you aren’t celebrating a New Year or an ‘Eid. The Earth’s companion gives all of us something to appreciate.

Looking back…

In case you were wondering about notable science events that occured the week following August 22…

On August 24, 79 A.D. Mount Vesuvius erupted, covering the cities of Pompeii (hopefully you had the chance to see the exhibit here in Houston at the Museum of Fine Arts), Herculaneum, and Stabiae under volcanic ash. The city was lost for 1,700 years – until it was accidentily rediscovered in 1748. The excavation of the city has given valuable insight into the city during the height of Roman Empire, acting as a time capsule, allowing scientists to study the buildings, food, and even people that were buried that fateful day.
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This is Mount Etna erupting in 2006 (there is no footage of the 79 explosion of Mount Vesuvius for obvios reasons.)

Also on August 24, in 2006, the International Astronomical Union (IAU) redefined the term “planet,” and Pluto was sent on its cosmic way (read the post about the controversy that ensued, by our astronomer James.) Pluto was “demoted” to the status of Dwarf Planet. There are currently eight planets and four dwarf planets in our solar system. The new definition of a planet is a celectial body that meets the following criteria:
    (a) is in orbit around the Sun, 
    (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
    (c) has cleared the neighbourhood around its orbit.

Karoli looking foward
Creative Commons License photo credit: ckaroli

On August 25, 1609, Galileo Galilei demonstrated his first telescope to Venetian lawmakers. He was one of the first men to build a telescope, and did so without actually ever seeing one of the few that existed. He was the first to discover any of Jupiter’s moons (he found 4), now known as the Galilean satellites.

On August 27, 2003, Mars made its closest approach to Earth in nearly 60,000 years. The last time Mars was that close to Earth, man had just began to migrate out of Africa. Man wouldn’t start settling down, farming, and beginning to live in cities for another 48,000 years. Mars passed approximately 34,646,416 miles (55,758,006 kilometers) from Earth.

Eight is Enough?

Creative Commons License photo credit: CommandZed

Two years ago this month, the International Astronomical Union adopted a new definition of ‘planet’ which excludes Pluto. Not only do I, as Planetarium Astronomer, continue to get questions about Pluto’s ‘demotion’, but scientists themselves continue to debate it. Right now (August 14-16, 2008), a conference called “The Great Planet Debate:Science as Process” is underway at the John’s Hopkins University Applied Physics Laboratory in Laurel, Maryland. The saga of Pluto and of the definition of ‘planet’ offers some insight into our solar system and into how science works.

northern tier sky
Creative Commons License photo credit: truello

The definition of ‘planet’ has changed before. Ancients looked at the sky and saw that certain ‘stars’ in the sky changed position, while most stars seemed to form the same patterns all of the time. The Ancient Greeks called the moving stars ‘planetes‘, or wanderers–this is the origin of the word. The Moon, too, appears near different stars each night. The Sun’s apparent motion is less obvious, since we don’t see the Sun and stars at the same time. Careful observers, however, can see that different stars rise and set with the Sun at different times of year. The full list of ‘planetes’, then, included the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. (Astrologers still use this archaic definition of planet).

Thanks to Copernicus and Galileo, people began to realize that the Sun, not the Earth, was the center of the solar system. The definition of ‘planet’ changed from ‘object which moves against the background stars’ to ‘object in orbit around the Sun’. The Sun and Moon, which had been planets, no longer were.

The position of Uranus, discovered in 1781, seemed to fit a pattern described by astronomers Johann Titius and Johann Bode. That same ‘Titius-Bode rule’ also predicted a planet between Mars and Jupiter, so when Giuseppe Piazza discovered Ceres at just the right distance in 1801, it was considered a planet. By 1807, four new ‘planets’ had been found between Mars and Jupiter (Ceres, Pallas, Juno, and Vesta). By the middle of that century, however, dozens of these new objects were being discovered; up to 100 had been found by 1868. It thus became clear that astronomers had in fact found a new category of solar system object. Astronomers adopted the term ‘asteroid‘, which William Herschel had recommended in 1802; ‘planet’ was redefined to exclude very small objects that occur in bunches. This is how science works; we must constantly revise even long standing definitions as we learn more about the universe around us.

In the late 19th century, astronomers noticed that Uranus and Neptune seemed to deviate ever so slightly from their predicted positions, suggesting that another planet was perturbing them. in 1906, Percival Lowell started a project to find the culprit, which he called ‘Planet X’. In 1930, Clyde W. Tombaugh located Pluto in sky photographs he took at Lowell Observatory in Arizona. It soon became apparent, however, that Pluto was not massive enough to influence the orbits of Uranus or Neptune. Throughout the mid 20th century, astronomers continued to revise Pluto’s estimated size downwards. From 1985 to 1990, Pluto’s equator was edge on to us, such that we saw its moon Charon pass directly in front of and behind Pluto’s disk. This allowed scientists to measure Pluto’s diameter more precisely, proving that it had not been the Planet X that Percival Lowell sought. Pluto’s diameter is just under 2400 km, a little less than the distance from the Rio Grande to the US/Canadian border. Pluto’s discovery, it turns out, was an accident.

In addition to small size, Pluto has an unusual orbit. Planetary orbits are ellipses rather than perfect circles. The eccentricity of an ellipse indicates how ‘out-of-round’ it is on a scale from 0 (perfect circle) to 1 (parabola–far end at infinity). Pluto’s orbit has an eccentricity of about 0.25, much greater than that of planets such as Earth (0.01) or Venus (0.007). The planets orbit nearly (but not exactly) in the same plane; Mercury‘s orbit, inclined by 7 degrees, is the most ‘out of line’. Pluto’s orbit, however, is inclined by 17 degrees.

Released to Public: Solar System Montage (NASA)

Behold: a pluto-less solar system.
Creative Commons License photo credit: pingnews.com

We divide the planets of our solar system into two categories: the inner planets (Mercury, Venus, Earth, and Mars) which are made mostly of rock, and the outer planets (Jupiter, Saturn, Uranus, and Neptune) which are gas giants with no solid surface. Pluto, however, fits in neither of these categories, as it is made of ice and rock (by some estimates, it’s 70% rock and 30% ice; by others, it’s about 50/50).

With its small size and abnormal orbit and composition, Pluto was always a misfit. Textbooks noted how Pluto fit in with neither the rocky inner planets nor the gas giants in the outer solar system. Still, Pluto remained a ‘planet’ because we knew of nothing else like it. There was simply no good term for what Pluto is.

That began to change in 1992, when astronomers began finding Kuiper Belt objects. The Kuiper Belt is a group of small bodies similar to the asteroid belt. Kuiper Belt objects (KBOs), however, orbit beyond Neptune’s orbit. Also, the Kuiper Belt occupies more space and contains more mass than does the asteroid belt. Finally, while asteroids are made mostly of rock, KBOs are largely composed of ice, including frozen ammonia and methane as well as water–just like Pluto. In addition to the Kuiper Belt proper, there is a scattered disc of objects thought to have been perturbed by Neptune and placed in highly eccentric orbits. Objects in the Kuiper Belt, scattered disc, and the much more distant Oort Cloud are together called Trans-Neptunian Objects (TNOs)

With the discovery of more and more KBOs, astronomers began to wonder if Pluto might fit better in this new category. Not only was the composition similar, but there is even a group of KBOs called plutinos, with orbits similar to Pluto’s. In the Kuiper Belt and the scattered disc, astronomers began to find objects approaching Pluto’s size, including Makemake, Quaoar, and Sedna.

Pluto can't get no respect
Pluto takes advantage of the wildly (?)
popular LOLcats to plead its case
with mankind.
Creative Commons License photo credit: the mad LOLscientist

To call Pluto a planet, but not these others, seemed arbitrary.

Finally, in 2005, a team of astronomers located Eris, which is slightly bigger than Pluto. Clearly, Eris and Pluto are the same kind of thing; either both are planets or both are not. If they both are planets, however, then should we include Quaoar et al., above? We have only just begun to explore and understand the Kuiper Belt and the scattered disc. Might we eventually find dozens of new ‘planets’ like Eris? Hundreds? Thousands?

This is what led the International Astronomical Union to reconsider the definition of ‘planet’ two Augusts ago. The IAU decided it was simpler to limit the number of planets to eight (Mercury through Neptune) and classify Pluto (and Eris, Quaoar, et al.) among the Trans-Neptunian objects. A new term, “dwarf planet,” includes the biggest asteroids and TNOs–those big enough to have assumed a spheroid shape. Still, other astronomers remain dissatisfied, hence the discussion going on in Maryland now.

There are two things we must keep in mind if we’re wondering when the Pluto question will be ‘resolved.’ First, decisions and conclusions of scientists are not holy edicts to be obeyed and never questioned. Quite the contrary, all such conclusions are provisional, pending new discoveries and better information. Any new decision reached this weekend is likely to be revised when the IAU meets again in 2009, and again in 2015 when the New Horizons mission arrives at Pluto. If it were any other way, science could not function.

Secondly, all categories which help us organize and understand things in our minds (including ‘planet’) are pure human inventions that only roughly correspond to nature. Although we need to categorize the things we see, nature does not; no matter how we classify objects, nature presents us with borderline cases that challenge us. Pluto is the same thing today as it was in 2005 or even before it was discovered in 1930. We need to distinguish our need for neat categories from our need to explore and describe nature.

Proud to be a space cadet? Learn more about astronomy:
Dust off your telescope – or visit the George Observatory – to see what’s in the night sky this month.
Ten billion trillion trillion carats – the universe has great taste in diamonds
If it blew a hole in your roof, you’re on the right track – how do you tell a rock might be a meteorite?