The Vernal Equinox, where the Sun crosses the Celestial
Equator (red line) on the first day of Spring (March 20/21).
Credit: Space.com/Starry Night

"Can you tell me about the upcoming beginning of the Age of Aquarius?" said the voice on the phone. "I heard that it starts this Saturday…"

Now, I get a lot of phone calls and emails from people with astronomy and sky related questions. Very often it's something like, "What was that thing that tried to land in my front yard yesterday evening?!" or, "Is it true that Mars will be closer to the Earth this weekend than it has been in a gazillion years?" I've even had one or two asking if it's true that the world is ending in 2012.

Okay, I'm embellishing a bit. Those are all very good questions, and I do my best to provide a science-based answer — like, "Venus tried to land in your yard," or "The Mars extra-close encounter happened in 2003… and it had only been less than a century since the previous time," or, "We'll just have to wait for 2012 to roll around to find out…"

As for the Age of Aquarius question, that got me to wondering. I've always regarded this issue as astrology-related more than astronomy, but I also realized there are physical underpinnings to the definition. So I fired up Google and clarified some of the details for myself. The first thing I learned is that, among astrologers at least, there is little agreement on precisely when the Age of Aquarius is supposed to begin (or if it's already begun). Different astrologers at different times and from different parts of the world have tried to define this, resulting in multiple schools of thought on the subject.

But from a purely astronomical standpoint, the delineation of these Ages is based on a natural physical cycle, just as a year is defined by Earth's motion around the Sun and a day is defined by Earth's rotation on its axis.

An astrological Age (aka "Great Year") is determined by the position of the Vernal Equinox — at least by one of the schools of thought… The Vernal Equinox is that point in the sky occupied by the Sun when it crosses the Celestial Equator heading into the Northern Hemisphere. So, you can think of the Vernal Equinox as a distinct point on the sky (and it's easy to locate on the first day of Spring: Just look at the Sun — I take that back: DON'T look at the Sun!)

But the position of the Vernal Equinox shifts over time due to a cycle of change in the orientation of the Earth's rotation. The Earth spins like a top, but also like a top it undergoes a gyrating motion, called precession. One complete gyration takes about 26,000 years — so all of the points in the sky defined by Earth's spinning (the celestial poles and equator, and, yes, the Vernal Equinox) move around the sky over 26,000 years.

At this moment, the Vernal Equinox is in the constellation Pisces — at least, within the region of the sky defined by modern astronomers as encompassing all the stars of Pisces. So, if one were to acknowledge the constellation boundaries according to modern astronomers, then one would say that we are in the Age of Pisces still (and, by the same definition of constellation boundaries, the Vernal Equinox will remain in Pisces until about the year 2600, when it will cross the border into Aquarius.)

However, there is little agreement among different groups of astrologers on where one constellation ends and another begins–and to my knowledge none of them have adopted the modern astronomical boundaries.

So, when does the Age of Aquarius begin? Depends on who you talk to….

A limb shot of Mercury's horizon taken by the
MESSENGER spacecraft on January 14, 2008.
Photo Credit "NASA/MESSENGER"

If you can take a name like "Mercury Surface, Space Environment, Geochemistry and Ranging" and craft it into a neat acronym like MESSENGER, then you may have a future working with NASA….

And no, this blog isn't about NASA acronymizations, but rather the heat-resistant robot behind one of them. MESSENGER is the space probe that NASA sent to Mercury to give the Solar System's innermost planet the first up-close look since 1975, when Mariner 10 flew by.

Though MESSENGER's main mission will begin in earnest when it returns to Mercury and finally settles into an orbit around the planet, on March 18th 2011, we were given a tantalizing peak last January 14th when the probe made its initial flyby.

What did this quick, on the fly snapshot tell us that we didn't know before? Well-a lot, considering Mercury has been one of the least understood planets in the Solar System, and was for a long time thought to be similar in character to our own Moon. Mercury is shaping up to be a lot less like Earth's Moon than its gray, cratered, airless appearance would mislead.

One key difference: density-how much material is packed into the planet; or how heavy a standard sized chunk of it would be. Our Moon is a lightweight on this score, with an average density of only 3.4 grams per cubic centimeter, while Mercury weighs in at a hefty 5.427 g/cc-almost as dense as Earth.

Another key difference: magnetic field. Planets like Earth and the Gas Giant worlds (Jupiter et al) generate respectable magnetic force fields, useful for everything from deflecting plasma flowing from the Sun (the "solar wind") to properly directing magnetic compass needles. Venus, Mars, and our Moon do not possess magnetic fields worth mentioning, as it turns out.

Mercury, on the other hand, does. Planetary magnetic fields are believed to be generated by currents in a planet's liquid outer core-like how the electric current in the wire coil of an electromagnet generates a magnetic field. Mercury's magnetic field suggests it still has some activity in its core-molten metals circulating in currents as the core slowly cools off. And speaking of Mercury's core, it appears to comprise 60% of the planet's mass-about twice what is "typical" for Terrestrial (solid) planets.

I've often imagined Mercury to be a cosmic goldmine, with its apparent richness in metals and its density. I wonder if an astronaut could just walk along and pick up chunks of gold from its surface….

Another interesting find by MESSENGER is that some of the flat plains on Mercury may have been formed by volcanoes, long ago. In particular, MESSENGER imaged a number of volcanoes along the edge of the Caloris Basin, a large impact basin-one of the largest in the Solar System, at 1550 kilometers across.

The news coming out of the innermost region of the Solar System makes me giddy. Too bad I have to wait until 2011 for my next look at Mercury. These things take time.

Asteroid 35107, captured on Chabot Space
& Science Center’s telescope.

Photo By Conrad Jung and Gerald McKeegan

You must be very quiet; we are hunting…asteroids!

On July 14th, 2008, an almost Hollywood-like drama took place in space nearby: a "double," or binary, asteroid whizzed past Earth, grazing by at a distance of only 1.4 million miles. One of the rocks is over 200 meters across, the other a whopping 600 meters-about half the size of Half Dome in Yosemite!

1.4 million miles may sound like a large distance, but by the standard of big rocks flying by the Earth, that's breathtakingly close. Discovered only last January, this pair of asteroids went from being completely unknown to blasting by Earth's doorstep in only months. Had they actually hit the Earth, they would have caused major devastation at and near the impact site, with very little warning.

Fortunately, there are programs to search for and track these flying mountains-also called "Near Earth Objects" (NEOs)-and I'm very pleased to announce that Chabot Space & Science Center (specifically our 36-inch reflecting telescope, "Nellie") has very recently become an official contributor to the NEO search program of the International Astronomical Union's Minor Planet Center (MPC)! Nellie is designated by the MPC as Observatory G58.

In this MPC program, observatories around the world contribute by searching for and tracking NEOs: asteroids, and comets, whose orbits can carry them close to Earth and which are large enough to cause catastrophic damage should they hit us.

In order to take part in the NEO program, Chabot observers Conrad Jung (on the Chabot staff) and Gerald McKeegan (of the Eastbay Astronomical Society) conducted a four-month program to develop and hone the necessary skills and data processing techniques, as well as to configure telescope equipment, to meet MPC qualifications.

To that end, they observed a set of known asteroids-some NEO's and some "Main Belt" asteroids. (One of these Main Belt asteroids, "Carter 10683," was named for former Chabot board member and president of the Eastbay Astronomical Society, Carter Roberts, who, sadly, passed away earlier this year.)

Chabot's asteroid hunters will begin their tenure of official asteroid observation by verifying the orbits of recently discovered NEOs and reporting the additional observations to the MPC, where it will be used to refine our knowledge of the NEOs' orbits. The next step in the program will ultimately be to hunt for currently undiscovered asteroids.

The process for finding, tracking, and reporting NEO observations goes something like this. With a digital (CCD) camera attached to the telescope, a section of the sky is imaged three or four times in a half-hour period. The images are processed and compared, and any star-like dots that are found to move between one image and the next become suspect asteroids. (The word "asteroid," by the way, literally means "star-like"-so named because through most telescopes asteroids are too far away and too small to appear as anything more than points of light.)

The coordinates of any moving dots are calculated for all of the images they are in, and this information is sent to the MPC to be added to the data from other NEO hunting observatories. From the combined observations of all the observatories, a precision database of the orbits of near-Earth rocks is maintained, and with it NEOs that may pose a threat to the Earth may be identified.

Hunting NEOs may be like searching for needles in a really big haystack-but in jobs like this, the more eyes on the problem the better. Nellie is now one more eye on lookout duty…

Click here for a closer view of the asteroid shown above.