IDA Information Sheets

I recently received a membership renewal notice from the International Dark-Sky Association (IDA) quoting Christopher Kyba that if light pollution continues to grow at the rate it currently is, “Orion’s belt will disappear at some point.”

This made me remember that I had written an IDA Information Sheet back in March 1997 that also had addressed how light pollution could erase much of the Orion constellation. I wrote,

Orion, arguably the most prominent of the constellations, begins to look more like “Orion, the Hunted” under a magnitude +4.0 sky. Under a magnitude +3.0 sky, Orion is on his deathbed. When light pollution is so bad that we have a magnitude +2.0 sky, only blazing Betelgeuse, regal Rigel, and Bellatrix and Alnilam remain to regale us.

Speaking of the IDA Information Sheets, I was the IDA Information Sheet Editor from 1996-1999, during which time I revised and edited most of the existing information sheets, edited and added many new ones from a number of contributors, as well as contributed many new ones that I authored, though I never credited myself as the author. One of the ones that I wrote was IDA Information Sheet 120, referenced above (and shown below). I have a complete hard copy set of IDA Information Sheets 1 through 175, the last of which was published in June 2000. I also have WordPerfect Macintosh source files for IDA Information Sheets 1 through 158, the last of which was completed on October 27, 1999.

Here’s IDA Information Sheet 120:

It is a shame that these IDA Information Sheets are no longer available anywhere on the Internet. At the very least, they are of historical interest, and I would say that much of the content is still relevant. Presumably, the IDA still has all of these information sheets, but after the Dave Crawford era, they have decided to remove access to them.

Finally, I want to express my disappointment that the International Dark-Sky Association has recently decided to change their name to DarkSky International. They are still in the process of changing everything over, but once that transition is complete, the IDA will be no more. The break with the Dave Crawford era will be complete. I, for one, will never forget how much Dave Crawford was able to accomplish during those early years, and how proud I was to have been a part of it.

The IDA/DSI is still a great organization, and I strongly encourage you to generously support it, as I do. It remains the most effective organization in the world addressing light pollution and the loss of our night sky and the natural nighttime environment.

Orion’s Throwing Stones!

Monday evening, October 21st, and Tuesday morning, October 22nd, will be the best time to watch the Orionid meteor shower, one of the year’s best meteor showers.

Up to two dozen meteors per hour might be seen between the hours of 3:00 a.m. and 5:00 a.m. or so— provided you can keep the 40%-lit waning crescent moon out of your field of view.

When to Watch:

10:16 p.m. Monday, October 21 through 12:19 a.m. Tuesday, October 22 (radiant rise in the ENE to moonrise)*

12:19 a.m. through 5:47 a.m. Tuesday, October 22 (moonlight will interfere; radiant will be highest in the sky at 5:18 a.m., and morning twilight begins at 5:47 a.m.)

Where to Be: In a rural area with no terrestrial lights visible that are brighter than the brightest star. Preferably no light domes (uncivil twilight) of cities or towns should be visible in the direction you will be looking.

What to Do: Dress for a temperature 20° F cooler than the actual air temperature. Bring a lawn chair and a warm sleeping bag or blankets. Try blocking the Moon with a building, hill, or trees— or use a strategically-placed black umbrella.

Where to Look: Generally look towards the radiant which is between Betelgeuse and the “feet” of Gemini.

What You’ll See: Fast meteors, many leaving persistent trains.

Meteor showers occur each year when the Earth in her orbit intersects the debris trail of a comet, and the comet that causes the Orionids is very famous, indeed. Halley’s Comet!

* Times listed are for Dodgeville, Wisconsin

Why are the Pleiades called the Seven Sisters?

The famous and beautiful Pleiades star cluster, which lies between 429 and 448 light years from us in the constellation Taurus the Bull, contains at least 2,109 stars that were formed around 125 million years ago—relatively recently on an astronomical timescale.  But when you look at the Pleiades with the unaided eye, unless you have unusually good vision and excellent sky conditions, you’ll see only six Pleiads.  If you see more than that, you’ll probably be able to see 8 or 9 Pleiads, maybe more.  But not seven.  So, why are the Pleiades called the Seven Sisters?

Here’s my conjecture.  Take a look at the Pleiades on a dark, moonless night. What do you see?  I think you’ll see a group of stars forming a tiny dipper shape, reminiscent of the much larger Little Dipper.

How many stars make up the Little Dipper shape?  Seven.  How many stars make up the Big Dipper shape?  Seven.  How many bright stars does nearby Orion have?  Seven.  Given this, and the fact that seven has long been considered a mystical number, it comes as no surprise, perhaps, that the Pleiades are called the Seven Sisters and not the Six Sisters or the Eight Sisters.  How many do you see?

The Pleiades will culminate1 at midnight for SW Wisconsinites on Friday night / Saturday morning, November 17/18.

1cross the celestial meridian; reach their highest point in the sky, due south

Bouy H., et al., 2015, A&A, 577, A148
Galli P. A. B., Moraux E., Bouy H., Bouvier J., et al., 2017, A&A, 598, A48
Stauffer J. R., Schultz G., Kirkpatrick J. D., 1998, ApJ, 499, L19

Eridanus Delights

The sixth largest constellation in the sky stretches from near Rigel on the west side of Orion down to 1st-magnitude lucida Achernar (declination -57°), a star that rotates so rapidly that its polar diameter is not even ¾ its equatorial diameter (Domiciano de Souza et al. 2014).  Achernar (α Eri) is appropriately named.  It means “The End of the River” in Arabic.

Eridanus, the River, contains two very special, easily seen, stars. 40 Eridani (also known as Keid and Omicron2 Eridani), a visual triple star system (magnitudes 4.4, 9.5, and 11.2) just 16.3 light years away, presents the most easily observed white dwarf star, 9.5-magnitude 40 Eri B, visible in any telescope.

A little further west we can find 3.7-magnitude Epsilon Eridani, the nearest star beyond the Alpha Centauri system thought to harbor one or more planets. Compared to our Sun, ε Eri is cooler (K2V), much younger (200-800 Myr), and somewhat metal-deficient (74% solar), and it is just 10.5 light years away. This youthful star still sports a dusty disk between radii 35 and 75 AU (Greaves et al. 1998), inside of which its putative planet, Epsilon Eridani b—at least 0.6 to 0.9 Jupiter masses—travels around the star in a highly elliptical orbit, completing one revolution every 6.85 to 7.26 years. At periastron, Epsilon Eridani b lies between 1.0 and 2.1 AU from its parent star, and at apastron, its distance is 4.9 to 5.8 AU (Mizuki et al. 2016). However, the existence of this or any other planets in the system is still far from certain, primarily due to the high level of photospheric activity that is difficult to disentangle from the radial velocity signals of any possible orbiting planets (Giguere et al. 2016).

Domiciano de Souza, A., Kervella, P., et al. 2014, A&A, 569, A10
Giguere, M. J., Fischer, D. A., et al. 2016, ApJ, 824, 150
Greaves, J. S., Holland, W. S., et al. 1998, ApJL, 506, L133
Mizuki, T., Yamada, T., et al. 2016, A&A, 595, A79