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.

The original Oakland Observatory in the 1880’s,
at Lafayette Square in Oakland. Credit: Chabot Space
& Science Center archives.This year marks an anniversary for the astronomical heritage of Oakland and the San Francisco Bay Area: Chabot Observatory turns 125!

Originally established as the Oakland Observatory in 1883, the facility was a unique creature from the very beginning. Conceived by then Oakland Public Schools Superintendent Jewett Gilson, who was inspired by a school observatory he saw in Philadelphia, the observatory was created for use by Oakland schools and the general public at large.

Gilson looked for, and eventually found, a donor to fund the observatory project: Anthony Chabot, a wealthy entrepreneur and philanthropist who made his fortune building municipal water systems in the Bay Area– including Lake Temescal and Lake Chabot. Anthony Chabot stipulated as part of his original $3,000 gift that the telescope shall forever be available for public observation at not cost– a tradition that continues today.

Chabot didn't want the observatory to be named for him, so in its earliest years it was called the Oakland Observatory. The public, as the story goes, insisted on calling it Chabot Observatory in gratitude for the gift– and eventually the name was made official.

The original location for the observatory and its 8-inch Alvan Clarke and Sons telescope ("Leah") was close to downtown Oakland in Lafayette Square– which today remains a square block of parkland, at 10th and 11th Streets and Martin Luther King Junior Way and Jefferson Street. In those days, 10 or so visitors on any given night would climb the tower-like structure to the telescope dome and peer at the heavens through the high quality instrument. Reservations had to be made in advance– sometimes as long as a month or two.

As Oakland grew, and particularly as it converted its street lighting from gas-powered lamps to electric lights, the necessity of moving the observatory to a darker spot grew. The observatory’s first director, Charles Burckhalter (who is said to have been the first person in Oakland with an astronomical telescope, set up in a backyard observatory at his home on Chester Street), arranged for the relocation. A number of different sites were considered– including a spot near Redwood Peak, the current location of the observatory– but a small hill next to the Mills College campus was finally adopted.

In 1915, Chabot Observatory opened at its new site, along with a new 20-inch Warner and Swasey telescope ("Rachel"), and continued to wow the public with the astronomical vistas it conveyed. In 1923 the directorship passed to Earle Linsley, a Mills College professor, who expanded the reach of the observatory to the public through outreach to schools and the establishment of an amateur astronomy group (today the Eastbay Astronomical Society).

Having visited this Chabot Observatory as a child in the 1960s, I now appreciate how long and distinguished a career those two telescopes spanned. At the time, I had no idea that Leah, even in 1968, was 85 years old-older than my grandparents! Then the observatory was run by the beloved Kingsley Wightman — "Mr. Science" to a generation or two.

It took the moving Earth to relocate the observatory a second time– literally. Because of Chabot Observatory’s location almost directly on top of the Hayward Fault, and the fact that the aging buildings were not quake– safe in the first place, another site had to be found: the present location of Chabot Space & Science Center, adjacent to Redwood Peak.

Happy 125th to Oakland’s special connection with the stars!

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

I had the privilege this week of interviewing Isabel Hawkins, an astronomer and director of the Center for Science Education at Berkeley's Space Sciences Laboratory. We talked about how people use evidence in science, how it is that we know what we know.

Hawkins isn't your ordinary astronomer. She began her career in an ordinary way: Ph. D. in Astronomy from UCLA, using mathematical models and computer simulations to give meaning to her observations. Along the way, she began to learn about how ancient people studied the sky. She's worked with us on our Ancient Observatories website, and hosted an equinox webcast from the top of the Mayan pyramid in the ancient astronomical site of Chichen Itza. And she's devoted a considerable amount of time and energy to understanding and appreciating how the knowledge of ancient people complements what modern scientists study today.

Most scientists today don't learn much about ancient knowledge. Observations such as measurements of the sun's movement across glyph-crusted temples don't usually meet the rigorous criteria of the scientific process: observe, create hypothesis, test, reproduce results.

In some instances, ancient people followed similar practices that were very similar to those used by modern scientists, observing things systematically and trying to devise explanations that will result in correct predictions. And sometimes the knowledge they gathered was, in fact, so "scientific" that modern researchers use it in their work today.

Take, for example, the knowledge of the Aymara Indians in Peru. The well-being of these adept weather-watchers was dependent on knowing how to time the planting of their vital potato crop with the arrival of the season's first rains sometime between October and December. They did this by making observations like meteorologists might today. They watched the Pleiades, or Seven Sisters constellation rise each night, and noted how fuzzy or clear it looked in the sky. Fuzziness caused by cirrus clouds high in the sky, meant rains were a ways off, and potato planting should be postponed. A clearly visible set of Sisters meant rains would come soon.

In 2002, Ben Orlove an environmental scientist at UC Davis, published a paper about the accuracy of the Aymara's observations of the Pleiades. It turned out that these ancient observations could be used by modern scientists to discern El Nino patterns in the past. Fascinating, since these measurements were taken long before there was a formal science of meteorology. Ancient knowledge becomes data points in modern research.

Hawkins cited another example: Ruth Ludwin, a seismologist at the University of Washington, has used generations-old folk tales of the Coast Salish Indians to help inform her computer modeling of earthquakes. The tales recount a serpent that knew where and when an earthquake would strike. By adapting location information from the stories into her computer models, Ludwin has found several small faults in the Seattle area that may have been active hundreds of years ago when the stories were created and may still pose a risk to local communities.

"It's interesting that what we call evidence can come in many forms," Hawkins says. "It might be part of a song, or a glyph writing or an artistic piece or a story."

And sometimes the records we keep and the stories we tell have more meaning than we can imagine when we create them.

Robin Marks is a journalist and science writer who current serves as a Multimedia Projects Developer for the Exploratorium in San Francisco, CA.