Metallicity

No, it’s not the name of a rock band. Astronomers (unlike everybody else) consider all elements besides hydrogen and helium to be metals. For example, our Sun has a metallicity of at least 2% by mass (Vagnozzi 2016). That means as much as 98% of the mass of the Sun is hydrogen (~73%) and helium (~25%), with 2% being everything else.

Traditionally, elemental abundances in the Sun have been measured using spectroscopy of the Sun’s photosphere.  In principle, stronger spectral lines (usually absorption) of an element indicate a greater abundance of that element, but deriving the correct proportions from the cacophony of spectral lines is challenging.

A more direct approach to measuring the Sun’s elemental abundances is analyzing the composition of the solar wind, though the material blown away from the surface of the Sun that we measure near Earth’s orbit may be somewhat different from the actual photospheric composition.  The solar wind appears to best reflect the composition of the Sun’s photosphere in the solar polar regions near solar minimum.  The Ulysses spacecraft made solar wind measurements above both the Sun’s north and south polar regions during the 1994-1995 solar minimum.  Analysis of these Ulysses data indicate the most abundant elements are (after hydrogen and helium, in order of abundance): oxygen, carbon, nitrogen, magnesium, silicon, neon, iron, and sulfur—though one analysis of the data shows that neon is the third most abundant element (after carbon).

The elephant in the room is, of course, are the photospheric abundances we measure using spectroscopy or the collection of solar wind particles indicative of the Sun’s composition as a whole?  As it turns out, we do have ways to probe the interior of the Sun.  Both helioseismology and the flux of neutrinos emanating from the Sun are sensitive to metal abundances within the Sun.  Helioseismology is the study of the propagation of acoustic pressure waves (p-waves) within the Sun.  Neutrino flux is devilishly hard to measure since neutrinos so seldom interact with the matter in our instruments.  Our studies of the interior of the Sun (except for sophisticated computer models) are still in their infancy.

You might imagine that if measuring the metallicity of the Sun in our own front yard is this difficult, then measuring it for other stars presents an even more formidable challenge.

In practice, metallicity is usually expressed as the abundance of iron relative to hydrogen.  Even though iron is only the seventh most abundant metal (in the Sun, at least), it has 26 electrons, leading to the formation of many spectral lines corresponding to the various ionization states within a wide range of temperature and pressure regimes.  Of the metals having a higher abundance than iron, silicon has the largest number of electrons, only 14, and it does not form nearly as many spectral lines in the visible part of the spectrum as does iron.  Thus defined, the metallicity of the Sun [Fe/H] = 0.00 by definition.  It is a logarithmic scale: [Fe/H] = -1.0 indicates an abundance of iron relative to hydrogen just 1/10 that of the Sun.  [Fe/H] = +1.0 indicates an abundance of iron relative to hydrogen 10 times that of the Sun.

The relationship between stellar metallicity and the existence and nature of exoplanets is an active topic of research.  It is complicated by the fact that we can never say for certain that a star does not have planets, since our observational techniques are strongly biased towards detecting planets with an orbital plane near our line of sight to the star.

References
Vagnozzi, S. 2016, 51st Recontres de Moriond, Cosmology, At La Thuile

The Zodiacal Light

Over the eons, as comets shed dust and asteroids collide, dust particles are freed from their parent bodies and, for a time, orbit independently around the Sun.  These tiny particles (typically 1 to 300 μm across) reflect sunlight that can be seen from Earth.  This phenomenon is called the zodiacal light (pronounced zoe-DYE-uh-cul).  It is a subtle yet beautiful cone of white light most easily seen extending up from the western horizon at the end of evening twilight, or projecting above the eastern horizon just before morning twilight begins.  This phenomenon is named after the zodiac because the dust is concentrated near the plane of the ecliptic.  The picture is complicated by the fact that there are zodiacal dust components that lie along the solar equatorial plane, the orbital plane of Venus, the invariable plane of the solar system, and the ecliptic.  All four of these reference planes lie within a few degrees inclination of each other.

Since the zodiacal light is generally brightest along the ecliptic just a few degrees away from the Sun, it is best to pick a time of year when that portion of the ecliptic is most nearly perpendicular to the horizon to make your observations.  This, of course, depends on your latitude (closer to the equator being better), but for those of us here in the Midwest, February, March, and April offer the very best times to see and photograph the zodiacal light above the western horizon at the end of evening twilight.  The very best times to see and photograph the zodiacal light above the eastern horizon before the beginning of morning twilight occurs for us in August, September and October.

In the images below, the yellow line is the ecliptic.  A mid-month view for each month of the year, morning and evening, is shown for latitude 43° N.  Note that the best months for viewing evening and morning zodiacal light listed above show the ecliptic at the steepest angles relative to the horizon.

In this year of 2017, the best dates and times for observing the zodiacal light are listed below.  The sky must be very clear.  The specific times listed are for Dodgeville, Wisconsin.

2017 Begin End Direction
Sun. Feb. 12 7:03 p.m. 7:32 p.m. West
Mon. Feb. 13 7:05 p.m. 8:05 p.m. West
Tue. Feb. 14 7:06 p.m. 8:06 p.m. West
Wed. Feb. 15 7:07 p.m. 8:07 p.m. West
Thu. Feb. 16 7:08 p.m. 8:08 p.m. West
Fri. Feb. 17 7:09 p.m. 8:09 p.m. West
Sat. Feb. 18 7:11 p.m. 8:11 p.m. West
Sun. Feb. 19 7:12 p.m. 8:12 p.m. West
Mon. Feb. 20 7:13 p.m. 8:13 p.m. West
Tue. Feb. 21 7:14 p.m. 8:14 p.m. West
Wed. Feb. 22 7:15 p.m. 8:15 p.m. West
Thu. Feb. 23 7:17 p.m. 8:17 p.m. West
Fri. Feb. 24 7:18 p.m. 8:18 p.m. West
Sat. Feb. 25 7:19 p.m. 8:19 p.m. West
Sun. Feb. 26 7:20 p.m. 8:20 p.m. West
Mon. Feb. 27 7:22 p.m. 8:22 p.m. West
Tue. Mar. 14 8:40 p.m. 9:22 p.m. West
Wed. Mar. 15 8:42 p.m. 9:42 p.m. West
Thu. Mar. 16 8:43 p.m. 9:43 p.m. West
Fri. Mar. 17 8:44 p.m. 9:44 p.m. West
Sat. Mar. 18 8:46 p.m. 9:46 p.m. West
Sun. Mar. 19 8:47 p.m. 9:47 p.m. West
Mon. Mar. 20 8:48 p.m. 9:48 p.m. West
Tue. Mar. 21 8:50 p.m. 9:50 p.m. West
Wed. Mar. 22 8:51 p.m. 9:51 p.m. West
Thu. Mar. 23 8:52 p.m. 9:52 p.m. West
Fri. Mar. 24 8:54 p.m. 9:54 p.m. West
Sat. Mar. 25 8:55 p.m. 9:55 p.m. West
Sun. Mar. 26 8:56 p.m. 9:56 p.m. West
Mon. Mar. 27 8:58 p.m. 9:58 p.m. West
Tue. Mar. 28 8:59 p.m. 9:59 p.m. West
Wed. Mar. 29 9:27 p.m. 10:01 p.m. West
Thu. Apr. 13 9:23 p.m. 10:07 p.m. West
Fri. Apr. 14 9:25 p.m. 10:25 p.m. West
Sat. Apr. 15 9:26 p.m. 10:26 p.m. West
Sun. Apr. 16 9:28 p.m. 10:28 p.m. West
Mon. Apr. 17 9:29 p.m. 10:29 p.m. West
Tue. Apr. 18 9:31 p.m. 10:31 p.m. West
Wed. Apr. 19 9:33 p.m. 10:33 p.m. West
Thu. Apr. 20 9:34 p.m. 10:34 p.m. West
Fri. Apr. 21 9:36 p.m. 10:36 p.m. West
Sat. Apr. 22 9:38 p.m. 10:38 p.m. West
Sun. Apr. 23 9:39 p.m. 10:39 p.m. West
Mon. Apr. 24 9:41 p.m. 10:41 p.m. West
Tue. Apr. 25 9:43 p.m. 10:43 p.m. West
Wed. Apr. 26 9:44 p.m. 10:44 p.m. West
Thu. Apr. 27 9:46 p.m. 10:46 p.m. West
Sat. Aug. 19 3:24 a.m. 3:40 a.m. East
Sun. Aug. 20 3:26 a.m. 4:26 a.m. East
Mon. Aug. 21 3:27 a.m. 4:27 a.m. East
Tue. Aug. 22 3:29 a.m. 4:29 a.m. East
Wed. Aug. 23 3:30 a.m. 4:30 a.m. East
Thu. Aug. 24 3:32 a.m. 4:32 a.m. East
Fri. Aug. 25 3:33 a.m. 4:33 a.m. East
Sat. Aug. 26 3:35 a.m. 4:35 a.m. East
Sun. Aug. 27 3:26 a.m. 4:36 a.m. East
Mon. Aug. 28 3:38 a.m. 4:38 a.m. East
Tue. Aug. 29 3:39 a.m. 4:39 a.m. East
Wed. Aug. 30 3:41 a.m. 4:41 a.m. East
Thu. Aug. 31 3:42 a.m. 4:42 a.m. East
Fri. Sep. 1 3:44 a.m. 4:44 a.m. East
Sat. Sep. 2 3:45 a.m. 4:45 a.m. East
Sun. Sep. 3 3:47 a.m. 4:47 a.m. East
Mon. Sep. 4 4:36 a.m. 4:48 a.m. East
Mon. Sep. 18 4:07 a.m. 4:49 a.m. East
Tue. Sep. 19 4:08 a.m. 5:08 a.m. East
Wed. Sep. 20 4:10 a.m. 5:10 a.m. East
Thu. Sep. 21 4:11 a.m. 5:11 a.m. East
Fri. Sep. 22 4:12 a.m. 5:12 a.m. East
Sat. Sep. 23 4:13 a.m. 5:13 a.m. East
Sun. Sep. 24 4:15 a.m. 5:15 a.m. East
Mon. Sep. 25 4:16 a.m. 5:16 a.m. East
Tue. Sep. 26 4:17 a.m. 5:17 a.m. East
Wed. Sep. 27 4:18 a.m. 5:18 a.m. East
Thu. Sep. 28 4:20 a.m. 5:20 a.m. East
Fri. Sep. 29 4:21 a.m. 5:21 a.m. East
Sat. Sep. 30 4:22 a.m. 5:22 a.m. East
Sun. Oct. 1 4:23 a.m. 5:23 a.m. East
Mon. Oct. 2 4:24 a.m. 5:24 a.m. East
Tue. Oct. 3 4:26 a.m. 5:26 a.m. East
Wed. Oct. 18 4:43 a.m. 5:43 a.m. East
Thu. Oct. 19 4:44 a.m. 5:44 a.m. East
Fri. Oct. 20 4:45 a.m. 5:45 a.m. East
Sat. Oct. 21 4:46 a.m. 5:46 a.m. East
Sun. Oct. 22 4:48 a.m. 5:48 a.m. East
Mon. Oct. 23 4:49 a.m. 5:49 a.m. East
Tue. Oct. 24 4:50 a.m. 5:50 a.m. East
Wed. Oct. 25 4:51 a.m. 5:51 a.m. East
Thu. Oct. 26 4:52 a.m. 5:52 a.m. East
Fri. Oct. 27 4:53 a.m. 5:53 a.m. East
Sat. Oct. 28 4:54 a.m. 5:54 a.m. East
Sun. Oct. 29 4:55 a.m. 5:55 a.m. East
Mon. Oct. 30 4:57 a.m. 5:57 a.m. East
Tue. Oct. 31 4:58 a.m. 5:58 a.m. East
Wed. Nov. 1 4:59 a.m. 5:59 a.m. East
Thu. Nov. 2 5:27 a.m. 6:00 a.m. East

On the February, March, and April evenings listed above, you will see a broad, faint band of light extending upwards from the western horizon, sloping a little to the left, and reaching nearly halfway to the top of the sky.

On the August, September, and October mornings listed above, you will see a broad, faint band of light extending upwards from the eastern horizon, sloping a little to the right, and reaching nearly halfway to the top of the sky.

It is essential that your view is not spoiled by nearby lights or any city to the west (Feb-Apr) or east (Aug-Oct).  Give your eyes a few minutes to adjust to the darkness.  Slowly sweeping your eyes back and forth from southwest to northwest (Feb-Apr) or northeast to southeast (Aug-Oct) will help you spot the zodiacal light band.  Once spotted, you should be able to see it without moving your head.

On the February, March, and April evenings listed above, the zodiacal light is best seen right at the end of evening twilight, and remains visible for an hour or so after that.

On the August, September, and October mornings listed above, the zodiacal light is best seen about an hour or so before the beginning of morning twilight, right up to the beginning of morning twilight.

Enjoy!

Deep Penumbral Lunar Eclipse – Friday, February 10, 2017

The Moon is Full on Friday, February 10, but that’s not all.  It will plunge deeply into the penumbral shadow of the Earth, not quite touching the umbral shadow.  The penumbral shadow is the part of the Earth’s shadow where you would see the Earth partially eclipsing the Sun.  Normally, penumbral lunar eclipses are no big deal, as they are very difficult or impossible to discern, but this time you should be able to see a noticeable darkening of the full moon from left to right as the eclipse progresses towards maximum penumbral shading, and then brightening from lower right to upper left as the Moon exits the Earth’s penumbral shadow, as shown in this video.  Of course, how much of this you will be able to see will depend on both your local moonrise and when evening twilight ends.

Here are local circumstances for Dodgeville, Wisconsin:

Event Time Moon Altitude
Penumbral Eclipse Begins 4:34:14 p.m. below horizon
Penumbral Eclipse First Visible? 5:14 p.m.? below horizon
Moonrise 5:21:03 p.m. 0° @ 72° (ENE)
Sunset 5:25:43 p.m.
Civil Twilight Ends 5:55:09 p.m.
Nautical Twilight Ends 6:28:36 p.m. 11°
Maximum Penumbral Shading 6:43:54 p.m. 13°
“Dark Enough” 6:45:07 p.m. 14°
Astronomical Twilight Ends 7:01:33 p.m. 16°
Penumbral Eclipse Last Visible? 8:14 p.m.? 29°
Penumbral Eclipse Ends 8:53:29 p.m. 36°

For those of us in SW Wisconsin, I wouldn’t bother looking much before 6:30 p.m., because evening twilight is likely to be too bright.  The best time to look will probably be at 6:43 p.m., just a little over a minute before twilight ceases to become any real concern1.  Evening twilight officially ends at 7:01 p.m., and you will probably notice some shading on the Moon until about 8:14 p.m.

The Moon will be inching closer towards Regulus during the penumbral eclipse (and, in fact, all night long), so watch for that.

For the record, a penumbral eclipse this deep (when there wasn’t also a partial or total lunar eclipse) hasn’t happened since March 14, 2006 (which was even deeper), and won’t happen again until January 10, 2085, though we need only wait until January 31, 2018 and January 20, 2019 for the next two lunar eclipses and they will both be total lunar eclipses—far more impressive than any penumbral lunar eclipse could ever be.  We’ll be seeing only the beginning partial phases of the 2018 eclipse here because the eclipsed moon will be setting during bright morning twilight.  Fortunately, we’ll have a front-row seat to the entire 2019 eclipse as all of it will occur high in the sky after dark, with totality ending conveniently before midnight.

1My late friend Joe Eitter (1942-2014), who was the observatory manager at Iowa State University’s Erwin W. Fick Observatory during its entire existence, used to say that by the time the Sun got down to 15° below the horizon, it is “dark enough”.

Below the Lunar Surface

Between 1969 and 1972, a dozen human beings walked upon the surface of the Moon, amounting to a total lunar exploration time of 3d 8h 32m 26s.  Gene Cernan returned to the Lunar Module at 11:40:56 p.m. CST on December 13, 1972.  In the over 44 years since, no one has followed in his footsteps.  Sadly, Gene Cernan, who died in 2017, never lived to see another human walk on the Moon.  Ever since his Apollo 17 mission, he has held the dubious distinction of being the “last man to walk on the Moon”.

Only four of the twelve Apollo astronauts who walked on the Moon are still living.  Will humans return to the Moon before the last of them dies?

Buzz Aldrin (1930-)

Neil Armstrong (1930-2012)

Alan Bean (1932-2018)

Gene Cernan (1934-2017)

Pete Conrad (1930-1999)

Charlie Duke (1935-)

Jim Irwin (1930-1991)

Ed Mitchell (1930-2016)

Harrison Schmitt (1935-)

Dave Scott (1932-)

Alan Shepard (1923-1998)

John Young (1930-2018)

Fortunately, robotic explorers orbiting and landing on the Moon in recent years have made some discoveries that provide a new impetus for humans to return to the Moon.  One of those discoveries is evidence for sublunarean structures that could provide “roughed in” habitats for human settlement.

Mother Nature may have done us a great favor thanks to lunar volcanism.

Most volcanism on the Moon occurred between 3 and 4 billion years ago when the lunar maria formed.

Sinuous rilles provide evidence of past volcanic flows and may be the collapsed remains of lava tubes.  There is some evidence to support that both Vallis Schröteri and Rima Sharp extend below the lunar surface as uncollapsed lava tubes.  Vacant lava tubes beneath the lunar surface may be quite common.

Vallis Schröteri
Vallis Schröteri (Schroter’s Valley) – More Beneath the Surface?
Rima Sharp
Rima Sharp – More Beneath the Surface?

Volcanism on the Moon may have continued almost up to the present day.  Not only do numerous small volcanoes on the Moon suggest active volcanism within the past 50 to 100 million years, but the irregular mare patch Ina in Lacus Felicitatis (“Lake of Happiness”) may be a volcanic feature no more than 10 million years old.

Ina
An Irregular Mare Patch (IMP) named Ina, in Lacus Felicitatis – Evidence of Geologically Recent Volcanism?

Could there be vacant lava tubes beneath the lunar surface?  On Earth, underground lava tubes can be found in Hawaii, Iceland, and many other locations around the world.

Thurston Lava Tube at Hawaii Volcanoes National Park, Big Island, Hawaii

Further evidence that there may be caverns and vacant tubes underneath the lunar surface are the many deep pits that have been discovered—over 150 so far.  Some of these pits may be openings into lava tubes beneath the surface, known as skylights.

Marius Hills Hole (MHH) – Lava Tube Entrance
Images of the Marius Hills Hole as observed under different solar illumination conditions by the SELENE/Kaguya Terrain Camera and Multiband Imager [JAXA/SELENE]
Mare Ingenii Hole (MIH) – Lava Tube Entrance?
Mare Tranquillitatis Hole (MTH) -Lava Tube Entrance?

NASA’s Gravity Recovery and Interior Laboratory (GRAIL) lunar orbiters mapped the gravitational field of the Moon in unprecedented detail, uncovering evidence of voids beneath the lunar surface.  Ground-penetrating radar, gradiometric, and gravimetric measurements are now needed to confirm the nature of these voids and whether they would be suitable structures for human habitation, shielding lunar residents from radiation, temperature extremes, and micrometeorites.

The Marius Hills Hole (MHH), about 160 ft. wide and 160 ft. deep at 14.100˚N, 303.262˚E, has been identified as leading to an intact lava tube below the lunar surface (Kaku et al. 2017).  The discovery was made after analyzing data from the Lunar Radar Sounder (LRS) instrument aboard the SELENE spacecraft.  LRS was an 800-watt ground penetrating radar, sweeping between 4 and 6 MHz every 200 μsec.  Each time these radio waves hit subsurface boundaries between rock and void, they reflected back towards the spacecraft and the lag times were used to estimate the depth and size of the voids.

Additional lava tubes or cavernous voids are thought to exist in the Marius Hills region, 13.5-13.8˚ N, 302.5-302.8˚ E.

The Lunar Advanced Radar Orbiter for Subsurface Sounding (LAROSS) mission has been proposed (Sood et al. 2016), and Gedex Inc., a Toronto-based geophysics company, is developing  a rover-mounted gravimeter and gravity gradiometer (Urbancic et al. 2015).  A gradiometer will be used to study the near-surface environment, and a gravimeter will go deeper.

These are exciting times for lunar exploration!

References
Beatty, K. 2014, (Geologically) Recent Volcanoes on the Moon?, Sky &    Telescope blog, October 14, 2014
Blair, D. M., Chappaz, L., Sood, R. et al. 2017, Icarus, 282, 47:55
Kaku, T., Haruyama, J. et al. 2017, Geophysical Research Letters, 44
Sood, R., Melosh, H. J., Howell, K. 2016, 26th AAS/AIAA Space Flight    Mechanics Meeting, AAS 16-464
Sumner, T. 2017, Science News, 191, 1, 5 (January 21, 2017)
Urbancic, N., Stanley, S., Ghent, R., et al. 2015, 46th Lunar and Planetary    Science Conference #1616

WISEA J045921.21+154059.2: A Bright, Nearby Infrared Dwarf

Are you up for an observing challenge?  If you have a telescope equipped with a CCD camera and an infrared filter, you might be able to detect the brightest known L-type infrared dwarf star WISEA J045921.21+154059.2.  (This author prefers to use the term “infrared dwarf” rather than the more popular caconym brown dwarf.)

WISEA J045921.21+154059.2, also known as WISE J045921.20+154059.4 and 2MASS J04592088+1541054, is a high-proper-motion star located not far from Aldebaran in the constellation Taurus at α2000 = 4h 59m 20.89s, δ2000 = +15° 41′ 05.42″.  This cool star has a spectral classification of sdL0.  Though its parallax has not yet been measured, its high proper motion may indicate it’s a star just a few light years away.  The “sd” classification on the L0 spectral type indicates that it is a subdwarf star—underluminous in comparison with a “normal” L0 star.

Infrared dwarfs—as their name implies—radiate mostly in the infrared portion of the spectrum rather than at visible wavelengths.  You can see this in the apparent magnitudes listed below.  Remember, the lower the number the brighter the star is at that wavelength/passband.

g = 20.08 ± 0.03
(PS1 g magnitude: center wavelength 4866 Å; green light)

r = 18.70 ± 0.01
(PS1 r magnitude: center wavelength 6215 Å; red light)

i = 17.14 ± 0.01
(PS1 i magnitude: center wavelength 7545 Å; infrared)

z = 16.49 ± 0.01
(PS1 z magnitude: center wavelength 8679 Å; infrared)

y = 16.19 ± 0.01
(PS1 y magnitude: center wavelength 9633 Å; infrared)

J = 14.96 ± 0.03
(2MASS J magnitude: center wavelength 12,350 Å; infrared)

H = 14.61 ± 0.06
(2MASS H magnitude: center wavelength 16,620 Å; infrared)

Ks = 14.30 ± 0.06
(2MASS Ks magnitude: center wavelength 21,590 Å; infrared)

W1 = 14.09 ± 0.03
(AllWISE W1 magnitude: center wavelength 34,000 Å; infrared)

W2 = 13.85 ± 0.04
(AllWISE W2 magnitude: center wavelength 46,000 Å; infrared)

W3 = 11.86 or brighter
(AllWISE W3 magnitude: center wavelength 120,000 Å; infrared)

W4 = 8.99 or brighter
(AllWISE W4 magnitude: center wavelength 230,000 Å; infrared)

References
Best, W. M. J., Magnier, E. A., et al. 2017, arXiv:1701.00490 [astro-ph.SR]

Prime Meridians

The choice of the prime meridian (0° longitude) is, of course, completely arbitrary.  Here in the U.S., it is not uncommon to find 18th & 19th century maps and navigational aids showing the prime meridian going through Philadelphia or Washington, D.C.  In October 1884, the International Meridian Conference convened in Washington, D.C.  At that conference, 22 of 25 nations voted to make the longitude line through Greenwich, England the internationally recognized Prime Meridian (0° longitude). Santo Domingo voted against the resolution, and France and Brazil abstained.

Some Bright and Deep Eclipsing Binaries

Here are the four brightest eclipsing binaries north of declination -30°, in order of brightness:

Menkalinan Algol Mintaka Alphecca
Beta Aurigae Beta Persei Delta Orionis Alpha Coronae Borealis
5h 59m 32s 3h 08m 10s 5h 32m 00s 15h 34m 41s
+44° 56′ 51″ +40° 57′ 20″ -00° 17′ 57″ +26° 42′ 53″
1.89 – 1.98 2.12 – 3.39 2.14 – 2.26 2.21 – 2.32
0.09m  3.96d 1.27m  2.87d 0.12m  5.73d 0.11m  17.36d

The first line is the proper name of the star.
The second line is the Bayer designation.
The third line is right ascension (epoch 2000.0).
The fourth line is the declination (epoch 2000.0).
The fifth line is the range of visual magnitude.
The sixth line is the Δm and period in days.

Honorable mention: two eclipsing binaries, one along the Vela/Carina border and visible only from latitudes south of 35° N; the other experiences an eclipse almost as deep as Algol.

δ Vel Sheliak
Delta Velorum Beta Lyrae
8h 44m 42s 18h 50m 05s
-54° 42′ 32″ +33° 21′ 46″
1.96 – 2.36 3.25 – 4.36
0.40m 45.15d 1.11m 12.94d

And, here are four reasonably bright eclipsing binaries with deep eclipses, north of declination -30°:

V Sge
AC UMa
SY Cyg
UW Vir
V Sagittae AC Ursae Majoris SY Cygni UW Virginis
20h 20m 15s 8h 55m 54s 19h 46m 34s 13h 15m 21s
+21° 06′ 10″ +64° 58′ 14″ +32° 42′ 18″ -17° 28′ 17″
8.60 – 13.90 10.30 – 14.00 10.70 – 14.20 9.00 – 12.40
5.30m 0.51d 3.70m 6.85d 3.50m 6.01d 3.40m 1.81d

Some eclipsing binaries have very long periods between minima.  Epsilon Aurigae (27.1 years), Zeta Aurigae (2.7 years), and Zeta Tauri (132.97 days) are examples.

References
Catalogue of eclipsing variables. Version 2 (Avvakumova+, 2013)

What’s That You Said…Both an Evening and a Morning Comet??

Sun-hugging Comet McNaught (C/2006 P1) was a wonderful sight ten years ago this month for the few who saw it in the northern hemisphere during January 2007.  It became visible to the unaided eye here in Wisconsin around January 4th, and brightened significantly during the next several days.  This unexpectedly bright comet reached a close perihelion (0.17 AU) on Friday, January 12, 2007 and became a spectacular sight from the southern hemisphere, but at that point our turn was over.

You may have heard (or witnessed) that Comet McNaught was visible in both the morning and evening twilight sky.  In fact, from SW Wisconsin the comet was visible both morning and evening from December 18th through January 9th.  How could that be?  It seems to defy common sense!

By looking at this video, you can see that Comet McNaught rose above and to the left of the Sun in the a.m. and set above and to the right of the Sun in the p.m.  Because the Comet-Sun line was nearly perpendicular to the ecliptic, as the sky rotated (due to the Earth’s rotation) during the day, Comet McNaught stayed “above” the Sun all day long, as shown in this video.  In the video, the blue/green line is the ecliptic, the plane of the Earth’s orbit.  Let’s use a clock analogy.  The Sun is at the center of the clock and Comet McNaught is at the end of the hour hand.  When Comet McNaught rises, it is at about the 10 o’clock position.  As the Sun rises and crosses the sky from SE to SW, the comet hour hand “moves” from the 10 o’clock position to the 2 o’clock position at sunset.  Though, of course, the clock itself is rotating clockwise, and the hour hand doesn’t move!

I’m not satisfied with this incomplete explanation, but at least you can see what is going on.  How good are you at visualizing spherical geometry in your head?  I’ll bet Stephen Hawking can do it.  If you can come up with a better description of this phenomenon—which will occur for any celestial object in the right position as seen from a certain range of latitudes— please share in a comment here!

Epoch and Equinox

We use the term epoch (of a given date) to refer to the actual measured coordinates of a star that takes into account precession, nutation, and proper motion. The term equinox means that the coordinates have been precessed to a given date, but that other factors affecting a star’s position have not been applied. So, equinox 2000.0 is not the same as epoch 2000.0.

Example: Barnard’s Star

Epoch 2000.0 coordinates: α = 17h 57m 48.49803s, δ = +4° 41′ 36.2072″ (the actual position of Barnard’s Star at 0h UT on January 1, 2000, accounting for precession, nutation, and proper motion)

Equinox 2017.1 coordinates: α = 17h 58m 39.20689s, δ = +4° 41′ 33.5614″ (coordinates have been precessed from epoch 2000.0 above to today’s date, but nutation and proper motion have not been applied)

Epoch 2017.1 coordinates: α = 17h 58m 37.85s, δ = +4° 44′ 37.8″ (the actual position of Barnard’s Star on January 19, 2017, accounting for precession, nutation, and proper motion)

Sometimes, the epochal coordinates are further adjusted to account for aberration and atmospheric refraction.  The latter tends to “lift” stars towards the zenith—the closer to the horizon the greater the lift.

Eugène Delporte and the Constellation Jigsaw

Belgian astronomer Eugène Joseph Delporte (1882-1955) discovered 66 asteroids from 1925 to 1942, but he is best remembered for determining the official boundaries of the 88 constellations, work he completed in 1928 and published in 1930.  The constellation boundaries have remained unchanged since then.

The International Astronomical Union (IAU), founded, incidentally, in Brussels, Belgium in 1919, established the number of constellations at 88—the same number, coincidentally, as the keys on a piano—in 1922 under the guidance of American astronomer Henry Norris Russell (1877-1957).  The IAU officially adopted Delporte’s constellation boundaries in 1928.

All the constellation boundaries lie along lines of constant right ascension and declination—as they existed in the year 1875. Why 1875 and not 1900, 1925, or 1930? American astronomer Benjamin Gould (1824-1896) had already drawn up southern constellation boundaries for epoch 1875, and Delporte built upon Gould’s earlier work.

As the direction of the Earth’s polar axis slowly changes due to precession, the constellation boundaries gradually tilt so that they no longer fall upon lines of constant right ascension and declination. Eventually, the tilt of the constellation boundaries will become large enough that the boundaries will probably be redefined to line up with the equatorial coordinate grid for some future epoch. When that happens, some borderline stars will move into an adjacent constellation. Even now, every year some stars change constellations because proper motion causes them to move across a constellation boundary. For faint stars, this happens frequently, but for bright stars such a constellation switch is exceedingly rare.