Bumps and ripples in the otherwise flat ring system of Saturn cast long shadows at equinox. Image credit: NASA/CassiniImagine a vast, flat plain spreading out before you for tens of thousands of miles in all directions, with no Earthly curvature to give the horizon its slightly finite look. Instead, it stretches seemingly to the infinite blackness of space in one direction, and slices straight into the streaky, wind-smoothed clouds of Saturn in the other…

Hard to imagine what it would be like to float just above the rings of Saturn, but what a sight it must be! As a kid, one of my favorite astronomical pass-times was imagining the view from other places in the Solar System.

Now imagine a towering bulge of frosty mist rising up out of this super-flat plane of ice chunks, literally the size of a mountain. Such is what was beheld by NASA's Cassini spacecraft last month–albeit, from a distance–when it turned its cameras to Saturn's vast rings during the few days surrounding Saturn's equinox (August 29, 2009), giving us a view never before seen.

Equinox on Earth, when the Sun is positioned directly over our equator, happens twice a year. Due to Earth's tilted rotational axis, as we orbit the Sun the latitude over which the Sun shines directly cycles north and south between the latitudes of the Tropics. On its way north to warm our (Northern Hemisphere) summers or south to leave us in the chill, the Sun crosses the equator on the equinoxes (Fall and Spring).

The same thing happens on Saturn, with two differences. First, Saturn takes nearly 30 years to orbit the Sun, so equinox comes only about every 14 years. Second, Saturn has its system of rings that encircle the planet directly above its equator, serving as a visible extension of the equator. At Saturn's equinox, the Sun is not only directly over the equator, but sunlight strikes the rings edge-on, like a flashlight shining on a flat piece of paper from the edge, the light just grazing over the surfaces on either side.

When this happens, any deviations from the flatness of the ring system—bumps and ripples–cast long shadows across the rings, making the features much easier to see. The same thing is seen on that piece of paper with shadows from creases and bumps leaping across the page.

As seen from Earth, equinox on Saturn means the rings appear to vanish as we look at them edge-on. This behavior puzzled astronomers long ago before they understood the rings for what they are. During the August 2009 Saturn equinox, however, for the first time in history we had a bird's-eye view of the rings during equinox, from Cassini. Cassini has been in orbit around Saturn for five years now.

Cassini spotted a number of prominent shadows trailing bright spots and ridges—bumps and ripples of different sorts rising above the ring plane.

Some of the bumps–icy ring material kicked up by the gravitational disturbance of a small moonlet inside the rings–were measured at over two miles high, the height of the Rocky Mountains. Other rippling features, such as long ridges running along the direction the rings encircle Saturn, are waves created by the gravity of moons orbiting outside the ring system. Still other types of disturbances observed are possibly caused by the impact of meteoroids or chunks of ice with the rings.

Saturn's rings are tens of thousands of miles across, but are extremely thin—perhaps no thicker than the height of a four-story building! So a bump or ripple as high as a mountain is a big deal!

Ah, to be on Saturn, now that equinox is here…

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….

It's approaching that time of year again: Spring Equinox. The blaze in my home's interior hallway has been signaling this for the last week.

The shadow of Chabot's "solar clock" at noon
on the equinox produces a pattern of solid green
straddling the gnomonI noticed late in the afternoon a couple days ago that the windowless hallway where we hang all of our family photos was afire in a shaft of bright sunlight, entering a window in the adjacent bedroom. Only around Equinox (Spring or Fall), when the Sun sets about directly west, does this happen in my house. The rest of the year the Sun sets too far north or south for this window-and-hallway alignment to take place. It's a striking event because for only a few days of the year my normally dark hallway explodes with radiance.

Ancient cultures all around the world made use of the changing rise and set position of the Sun to track the seasons, and either observed special alignments of sunlight and shadow with geographical features, or built structures that made the special alignments. Stonehenge is one famous example, but there are plenty of other seasonal observatories in just about every part of the world.

Unlike the more distant stars in the sky, which always rise and set at the same points on the horizon, the Sun (a star too, of course) wanders northward and southward in the sky throughout the year, and so its rise and set points migrate. On the Equinoxes the Sun rises directly at the east point on the horizon and sets directly at the west point-but at Summer Solstice in the Bay Area it rises a full 30 degrees to the north, and at Winter Solstice 30 degrees to the south.

The reason for the Sun's annual wandering comes from the tilt of Earth's rotational axis with its orbit around the Sun. At our (Northern Hemisphere) Summer Solstice, our hemisphere is tipped toward the Sun and the Sun appears at its most northerly point in the sky; we receive more hours of sunlight and more direct rays from the Sun-so it's warmer. Winter Solstice is opposite, with our hemisphere tipped away and the Sun and the Sun farthest to the south, making for shorter hours of daylight and less direct solar rays–and so it's colder.

Equinox is a middle point between solstices: the Sun is poised between the northern and southern extreme points of the solstices-positioned directly over Earth's equator-and the hours of daylight and night are about equal.

Does your home or place of work function as a solar seasonal calendar, as mine does? Is there a special time of year when you notice a striking pattern of light and shadow, a special alignment of walls, windows, doors, or other features? From the location of Chabot Space & Science Center, at equinox the Sun sets directly on the Golden Gate Bridge… .

If you have noticed something like this, then you've experienced what many ancient peoples noticed about the seasonal changing of the Sun. Their observations led them to understanding, or at least making use of, the cycle of the Earth revolving about the Sun to establish the earliest calendar systems.

Take a look and see what you notice, especially around Equinox (March 19, Pacific Time-March 20 GMT).

Benjamin Burress is a staff astronomer at The Chabot Space & Science Center in Oakland, CA.