Alt-Azimuth – “Altitude and Azimuth”. This is about spherical coordinates in degrees and something to do with your telescope. EGGHEADS, ANGRILY COMMENT NOW. (Seriously, please use the comment section to weigh in and correct or explain anything that I’ve gotten wrong or am just making a flip joke about. Thanks. I’d love to hear from you.)

Annular eclipse—A type of partial eclipse where the disk of the moon is JUUUST a little too small to blot out the sun, causing a way cool “ring of fire” effect. Not safe for direct viewing without protective eyewear. Also referred to as a hybrid eclipse.

Aphelion—The point in Earth’s orbit when we’re furthest from the sun. Eclipses that occur near aphelion are total because the size of the sun in the sky is smaller, and the same size as the moon is near perigee.

Apogee—The further possible point the moon can orbit around the earth. The closer to apogee, the shorter the eclipse.

Baily’s beads—Ready to have your mind blown? Baily’s beads are the dots of light in a ring you see with the naked eye immediately before totality caused by SUNLIGHT SHINING THROUGH THE MOUNTAINS ON THE MOON.

Centerline—The position running through the middle of the path of totality where the eclipse is at maximum duration. Most chasers fight to be closest to the centerline; others interested in specific observations of the chromosphere, shadow bands and other associated phenomenon might set themselves up closer to the edge of the path.

Chaser— AKA umbraphile; one who uses valuable vacation time and travel dollars to stand in the shadow of a total solar eclipse, usually on multiple occasions. (Why aren’t we called lunatics?) A bizarre niche market for the travel industry. (I recommend hooking up with TravelQuest for this purpose.)

Chromosphere—The layer of the sun’s atmosphere just above the photosphere, visible as a red rim around the black disk during a total solar eclipse.

Contact—Points at which the disk of the moon touches the edges of the solar disk. A total solar eclipse can be described by four contact points: First, when the moon touches the edge of the sun. Second, totality, when the disk of the moon makes it all the way across the sun initiating the Diamond Ring. Third, the “outtro” Diamond Ring, ending totality. And fourth, when the tiniest final sliver of moon no longer bites into the disk of the sun.

Cookie bite—Description of the solar disk during the early partial phases. (Not to be confused with “orange peel”, the image of the sliver of sun close to totality when viewed through an orange solar filter.)

Corona—Meaning “crown”, the wispy plasma that appears as a white glow with long streamers reaching away from the surface of the sun. The solar corona can be seen with the naked eye only during a total eclipse. The coronal shape is unique to each eclipse, and varies due to sunspot activity.

Diamond Ring—The phenomenon visible at Second and Third Contact when the last point of light from the sun is blocked by the moon, creating an enormous, shiny diamond ring in the sky, just like you see in cartoons. Looks exactly as it sounds.

Duration—Though eclipses can drag on for two or so hours after First Contact (the beginning) to Fourth Contact (the end), duration usually refers to totality, the period between Second and Third Contact. The shortest duration can be seconds; the longest possible is slightly more than seven and a half minutes.

Eclipse—In general, when any heavenly body is visibly darkened when it falls into the shadow of another heavenly body. Eclipses can be annular, partial, total and lunar (an eclipse of the moon occurs when the shadow of the earth falls on the moon).

Eclipse glasses—Solar filters fashioned into lenses and set into (usually) inexpensive cardboard frames or sometimes more substantial eyewear. Often made of Mylar. Make sure they’re labeled as “CE” compliant, or choose welder’s goggles fitted with a #14 lens for safe, long term exposure and a fetching, sexy look.

Edge effects —A ground phenomenon that occurs when the sun is more than halfway obscured, causing tall shadows to appear crisp on one edge and indistinct on the other. I’ve never seen this.

Edging—Observing at the edge of the path of totality to prolong Diamond Ring viewing (but sacrificing duration).

First Contact—The official beginning of the eclipse—the first partial phase—when the moon begins to obscure the sun.

Fourth Contact—The official end of the eclipse; the final moment of the last partial phase when the moon ceases to block any part of the sun.

Horizon effect—AKA twilight glow. The 360° sunrise (or sunset, if you’re an Enneagram 4) occurring during totality as the moon’s shadow commingles with the atmosphere at the horizon.

Hydrogen alpha filter or “H-Alpha”—a solar filter for your telescope that allows safe viewing of the sun (and sunspots) during the partial phases due to something about nanometers.

Partial  solar eclipse—A relatively common occurrence, when the sun is partly obscured by the moon to any percent that isn’t 100% total. Was once fun to observe until you saw your first total. Now like kissing your sister.

Path—The track of the lunar shadow as it passes across earth during an eclipse; usually meaning the path of totality, in the umbra, where the total eclipse can be observed. “The path” varies in width up to only 100 to 150 miles wide at its widest point. The closer one stands to the center of the path, the longer the eclipse will last.

Penumbra—The outer part of the lunar shadow when the sun is only partially blocked. Those standing in the penumbra see a partial eclipse.

Perigee—The closest distance between Earth and a body in orbit around us. The disk of the moon appears largest in the sky—and a total eclipse is possible—when the moon is at perigee.

Perihelion—When the orbit distance between the sun and Earth is as close as possible. Eclipses during perihelion are shorter in duration.

Photosphere—The layer of the sun’s atmosphere just below the chromosphere that causes “sunlight”. It’s very hot.

Pin hole projection—A fun way to ground-observe the partial phases of a solar eclipse. Poke a small hole into a piece of paper and angle it so that it casts a tiny shadow on the ground in the image of the “cookie bite”.

Prominence—The bright red flames that erupt from the edge of the sun’s photosphere into the corona, visible during totality. Solar magnetic fields may form a “prominence loop”.

Saros cycle —The ancient, still-used repeating cycle to calculate the occurrence of eclipses. Specifically? Every 18 years, 11 days, and 8 hours per cycle. A 56-year “exeligmos cycle” is comprised of three saros cycles (with four eclipses in the same saros). Related: the metonic cycle, a period of 19 years, 6,932.4 days, during which it’s possible for a series of up to five eclipses to occur on the same date 19 years apart. And so forth. I was told there would be no math.

Second Contact—The moment the moon fully obscures the sun, creating totality.

Shadow bands—A weird, not-always-observed (and not-fully-explained) ground phenomenon during the last moments before totality and directly after caused by “shimmers” in the atmosphere. Some describe shadow bands as similar to the undulating patterns on the bottom of a swimming pool. Best seen on a flat, light colored surface, like a concrete patio or wide area of beach.

Sunspots—Dark blemishes on the sun where the photosphere is cooler. Observable through a telescope with a solar filter. Tracking sun spot activity can help predict of the quality and shape of the corona and prominences.

Streamers—The long white projections of the corona typically at the sun’s equator…but length, placement, and shape of the corona will be a glorious surprise (see sunspots, above).

Telescope—You know what this is. A telescope is not necessary to enjoy a total eclipse. But it’s fun if there’s a good one nearby (owned by an seasoned operator) that you can peek through.

Third Contact—Sadly, the moment when totality ends and the sun reemerges from behind the moon.

Total eclipse—When the sun is completely covered by the moon…or is it?  Here’s a question I’ve never found an answer to. Is “total eclipse” synonymous with totality, or does the term refer to the entire event from First to Fourth Contact, with the payoff of totality in the middle?

Totality—The period of time—up to 7 minutes, 32 seconds—when the sun is completely blocked by the moon.

Transit—An underwhelming phenomenon to observe during the off years when there’s no total eclipse on the calendar. Just kidding. Sort of. A celestial body is “in transit” when it passes in front of the sun and can be observed as a tiny speck from Earth as it crosses. Catch the Transit of Mercury in May of 2016—but it’s a long wait until the next Transit of Venus in 2117. (My profile picture was taken during the TofV in 2012.)

Umbra—The darkest part of any shadow where the light source is completely obscured. A small light, like a flashlight, forms only an umbra; a giant source (like the sun) forms both a penumbra and an umbra. Only in the umbra during a solar eclipse can you experience the astonishing awe of totality.

Umbraphile—One who loves eclipses. (You too? I’d love to hear from you—drop me a line in the comments. Clear skies!)