Observe the Equinox in Your Back Yard Last year during the fall equinox at Turner Farm we followed the tip of the shadow from our flagpole in the field adjacent to the Roll-Top and RATO observatories. Every half hour we put a stake in the ground where the shadow tip fell. Amazing...on the equinox the shadow follows a straight line from west to east. On other days of the year the shadow follows a bent (hyperbolic) path. You can see the stakes put into the ground at half hourly intervals form a line. At the top of the flagpole is a small ball. You would think that it is easy to look for its shadow and mark the shadow tip on the ground. But a strange thing happens. It is very much like a partial solar eclipse (and remember NEVER look directly at the sun). In this case it is the ball on the flagpole blocking part of the sun. Because they appear as different sizes, sunlight "leaks" around the flagpole ball and the shadow on the ground is washed out and indistinct. This is a penumbral shadow in technical parlance and makes the shadow hard to see. For our flagpole at a time near noon, the ball appears almost the size of the sun and the shadow is sharper and easier to see and mark with a stake. Now let’s go to a smaller scale, something that you can do in your backyard. We'll show you an example of what we did using the observatory's sidewalk where we have power piers that are just short of 3-feet high. On the equinox we marked the shadow tip with chalk at different times. The chalk is hard to see in the photo, so we enhanced it a bit with purple dots. The photo shows the morning shadow traveling west to east as the ten o'clock hour advances. The dots form a straight line. You can make the same observations using a stick in the ground and follow the tip of the shadow through the day, putting a pebble or a penny down at half-hour intervals. You too can then “draw a straight line” to show your equinox shadow’s travel. So why does the shadow move in a straight line? Imagine that you have a great big ball with its center at the top of your stick. This is the celestial sphere with its axis pointed to the north celestial pole. On the equinox the sun is on the celestial equator, sort of like a belt going around the middle of the sphere. During the day the sun travels from east to west on that belt as the whole celestial sphere turns. (Of course you and I know it is the earth turning, not the sun). But that celestial equator when projected to the ground is seen on edge. That means that it doesn't look like a circle, it looks like a line...a straight line. And that's what happens to the stick's shadow. It follows that projection following a straight line. At other times of the year the sun is either north or south of the celestial equator and no longer follows a "great circle" but appears to follow a small circle on the sphere. (The smallest circles...and you can see them on some globes of the earth ...are called the Tropic of Cancer and the Tropic of Capricorn for the sun's summer and winter solstices). Those circles when projected onto the ground form hyperbolas. During the summer the hyberbola shadow line curves downward to the south, and during winter the path of the shadow will have a morning and evening end that points upward toward the north. Only on the spring and fall equinox does the sun's daily (diurnal) shadow travels in a straight west-east line. Try it yourself on the equinox occuring on Tuesday, September 22, 2020.

Fall Astronomy Courses 2020 - Remote Learning Through the Fairfax County Park Authority Parktakes program, you can take an introductory course on telescopes, learn about the night sky, the daytime sky of sun, rainbows and arcs, the planets and the objects of our solar system, meteorites and meteor showers, the stars, constellations, and more. We will even put cameras on the telescopes so you have a front row seat of the universe from your computer or mobile app. In normal times, these courses are held at Turner Farm Park in the Roll-Top Observatory or on park grounds. Due to the pandemic, these courses will be presented as webinars. These virtual programs are hosted in on-line Zoom meetings by FCPA staff and volunteers of the Analemma Society. Fall Courses: Date Time Course Title Age Thu Sep 10 2020 7:30 PM Webinar Rainbows, Haloes, and Glories 6-Adult Tue Sep 22 2020 7:30 PM Webinar Introduction to Astronomy – Solar System 8-Adult Wed Sep 30 2020 7:30 PM Webinar Origin of Our Universe – The Big Bang 8-Adult Wed 10/7/2020 7:30 PM Webinar Nebulas, Star Clusters & Galaxies 8-Adult Tue 10/27/2020 7:30 PM Webinar Exploring the Moon 8-Adult Thu 10/29/2020 7:30 PM Webinar Introduction to Astronomy – Topic To Be Announced 8-Adult You can register at: http://www.fairfaxcounty.gov/parks/parktakes/. All it takes is the sign up with Parktakes, get your member number, locate the course registration page and provide a valid email address along with a small payment for the course using Visa or MasterCard. Go to the FCPA Parktakes website or www.analemma.org for course descriptions. The courses can be located via Parktakes Find A Program by searching for courses under “Virtual FCPA” in the place or park location box. Our astronomy topic webinars are under the “Science and Technology” category or activity. Instructions will be provided after you register on how to log-in for the webinars, so we advise registering for these programs in advance - - do not leave registration for the last minute. Course Descriptions: Family Fun: Rainbows, Haloes and Glories Webinar. Instructor Jeff Kretsch Webinar meets 7:30 PM Thursday September 10. Covers common and some uncommon phenomena people can see in the day or night sky such as rainbows, haloes and glories. We will also cover sunrise and sunset phenomena like the green flash, the purple light, and simple questions like why the sky is blue and why sunsets are orange. This simple course will teach people how to better appreciate the sky both day and night. After the main talk, we will hold a virtual sky tour on some of the constellations and objects visible in the night sky. Be sure to register at http://www.fairfaxcounty.gov/parks/parktakes/ Introduction to Astronomy Webinar Instructors Analemma Society Staff Webinar meets 7:30 PM Tuesday September 22 – The Night Sky and The Solar System Webinar meets 7:30 PM Thursday October 29 – The Night Sky plus Topic To Be Announced This course provides a general introduction to some fundamentals of astronomy and the universe we see in the sky around us. We will discuss the basic types of astronomical objects from the small to the large, the motions of the planets and stars, and interesting phenomena we can observe in the sky. The course covers different sub-topics on astronomy from class to class. After the main talk, we will hold a virtual sky tour on some of the constellations and objects visible in the night sky. Be sure to register at http://www.fairfaxcounty.gov/parks/parktakes/ Origin of Our Universe – The Big Bang Webinar Instructor Robert Kellogg, PhD. Webinar meets 7:30 PM Wednesday September 30. We all want to know how our universe began, how stars and galaxies evolved, and what is this stuff called dark matter and dark energy. In this seminar we will look at what astronomers try to answer these big questions about the big bang. We will also look at some of the astronomers and what their observations revealed. After the main talk, we may hold a virtual sky tour on some of the constellations and objects visible in the night sky. Be sure to register at http://www.fairfaxcounty.gov/parks/parktakes/ Nebulas, Star Clusters, & Galaxies Webinar! Instructor Cal Powell Webinar meets 7:30 PM Wednesday October 7. We are familiar with the stars and planets that we can see when the Sun sets. However, the night sky contains many wonders that are visible with binoculars or modest telescopes. This fun and interactive presentation is designed as an introduction to the most common of these astronomical “deep sky” objects. After the main talk, we will hold a virtual sky tour on some of the constellations and objects visible in the night sky. Be sure to register at http://www.fairfaxcounty.gov/parks/parktakes/ Exploring the Moon Webinar Instructor Bill Burton Webinar meets 7:30 PM Tuesday October 27. The Moon is as old as the Earth, and its surface is marked by craters and lava flows billions of years old. Explore these lunar features through high-powered telescopes and learn about their origins. See where the Apollo lunar landings occurred and discuss what the astronauts found. After the main talk, we may hold a virtual sky tour on some of the constellations and objects visible in the night sky. Be sure to register at http://www.fairfaxcounty.gov/parks/parktakes/ Attachments: FCPA Fall 2020 Parktakes Astronomy Webinars.pdf [ ] 372 kB