Stars Like Our Sun – II

Last time we looked at the brightest G2V stars in the nighttime sky.

Now, we’ll focus on a more sophisticated approach to identify stars that are most like our Sun.  A solar twin is currently defined as a star with the following characteristics (Adibekyan et al. 2017):

Teff = 5777 ± 100 K

log g = 4.44 ± 0.10 dex

[Fe/H] = 0.00 ± 0.10 dex

Teff is the effective temperature of the star.  The effective temperature is the uniform temperature of a black body (which stars closely approximate) that would have the same radiant energy at all wavelengths as the star.

log g is the surface gravity, the base-10 logarithm of the gravitational acceleration, at the photosphere of the star.  The surface gravity is presented logarithmically because the gravitational acceleration at the surface of a star ranges over many orders of magnitude depending on the type of star (for example, a red dwarf vs. a white dwarf or neutron star).

[Fe/H] is the metallicity of the star, giving the ratio of iron to hydrogen atoms in logarithmic units relative to the Sun.  So measured, metallicity as the iron content of a star’s photosphere is often a reasonable proxy for the total metal content of the star (all elements except for hydrogen and helium).

Looking at a recent list of 21 solar twins in the solar neighborhood (Nissen  2016), we find that HD 20782 has the closest Teff match to the Sun, HR 2318 has the closest log g match to the Sun, and HD 222582 has the closest [Fe/H] match to the Sun.  The star with the closest match to all three solar twin characteristics is 18 Scorpii.

HD 20782
Fornax
Teff = 5776K, log g = 4.345, [Fe/H] = -0.058
Age = 8.1 ± 0.4 Gyr, Mass = 0.97 M
mv = 7.38, mb = 8.03, B-V = 0.65, G1.5V
α2000 = 03h 20m 04s, δ2000 = -28° 51′ 15″
116 – 118 ly
Single star with one known planet, 1.4 – 2.4 MJ, 592d orbital period, in a highly eccentric orbit (e = 0.97).

HR 2318
Canis Major
Teff = 5871 K, log g = 4.445, [Fe/H] = 0.047
Age = 2.7 ± 0.5 Gyr, Mass = 1.05 M
mv = 6.39, mb = 7.01, B-V = 0.62, G1.5V
α2000 = 06h 24m 44s, δ2000 = -28° 46′ 48″
71 – 72 ly
Single star with one known planet, 87% the mass of Uranus, 5.89d orbital period, in a mildly eccentric orbit (e = 0.3).

HD 222582
Aquarius (below the Circlet of Pisces)
Teff = 5784 K, log g = 4.361, [Fe/H] = -0.004
Age = 7.0 ± 0.4 Gyr, Mass = 1.00 M
mv = 7.69, mb = 8.34, B-V = 0.65, G5V
α2000 = 23h 41m 52s, δ2000 = -05° 59′ 09″
136 – 140 ly
Single star with one known planet, 7.1 – 8.4 MJ, 572d orbital period, in a very eccentric orbit (e = 0.725).

18 Scorpii (18 Sco)
Scorpius (just below the “coffee pot” asterism of Ophiuchus)
Teff = 5809 K, log g = 4.434, [Fe/H] = 0.046
Age = 4.0 ± 0.5 Gyr, Mass = 1.03 M
mv = 5.50, mb = 6.15, B-V = 0.65, G5V
α2000 = 16h 15m 37s, δ2000 = -08° 22′ 10″
45.1 – 45.6 ly
Single star, very similar to our Sun.

An additional solar twin in the solar neighborhood has been added recently (Yana Galarza 2016): HD 195034.  It has an even closer match to the Sun’s [Fe/H] than HD 222582 does.

HD 195034
Vulpecula
Teff = 5818 K, log g = 4.49, [Fe/H] = -0.003
Age = 2.0 ± 0.4 Gyr, Mass = 1.03 M
mv = 7.09, mb = 7.74, B-V = 0.65, G5
α2000 = 20h 28m 12s, δ2000 = +22° 07′ 44″
91 – 92 ly
Single star.

References
Adibekyan, V., Delgado-Mena, E., Feltzing, S., et al. 2017, arXiv:1701.05737
Nissen, P.E. 2016, A&A, 593, A65
Yana Galarza, J., Meléndez, J., Ramírez, I., et al. 2016, A&A, 589, A17

Stars Like Our Sun

The spectral type of our Sun is G2V, that is to say, a G2 main-sequence star.

Sun
Zodiacal Constellations
mv = -26.75, mb = -26.10, B-V = 0.65
Ecliptic
0.0000158 ly
Single star

Here are the brightest stars visible in the nighttime sky that have the same spectral type and therefore are, arguably, most like our Sun.  All have an apparent visual magnitude brighter than +6.00.

Rigil Kentaurus A, Alpha Centauri A (α Cen A)
Centaurus
mv = 0.01, mb = 0.72, B-V = 0.71
α2000 = 14h 39m 36s, δ2000 = -60° 50′ 02″
4.30 – 4.34 ly
Triple star system

Alula Australis B, Xi Ursae Majoris B (ξ UMa B)
Ursa Major
mv = 4.73, mb = 5.38, B-V = 0.65
α2000 = 11h 18m 11s, δ2000 = +31° 31′ 46″
28 – 30 ly
Quintuple star system

HR 4523 A
Centaurus
mv = 4.88, mb = 5.55, B-V = 0.67
α2000 = 11h 46m 31s, δ2000 = -40° 30′ 01″
30.0 – 30.1 ly
Binary star system; exoplanet

Eta Coronae Borealis A & B (η CrB A & B)
Corona Borealis
A: mv = 5.577, mb = 6.123, B-V = 0.546
B: mv = 5.95, mb = 6.48, B-V = 0.53
α2000 = 15h 23m 12s, δ2000 = +30° 17′ 18″
57 – 59 ly
Triple star system

HR 8323
Grus
mv = 5.58, mb = 6.18, B-V = 0.60
α2000 = 21h 48m 16s, δ2000 = -47° 18′ 13″
51.9 – 52.5 ly
Single star

Mu Velorum B (μ Vel B)
Vela
mv = 5.59, mb = 6.10, B-V = 0.51
α2000 = 10h 46m 46s, δ2000 = -49° 25′ 12″
116 – 119 ly
Binary star system

HR 7845 A
Capricornus
mv = 5.65, mb = 6.34, B-V = 0.69
α2000 = 20h 32m 24s, δ2000 = -09° 51′ 12″
79 – 80 ly
Binary star system

HR 3578
Hydra
mv = 5.86, mb = 6.39, B-V = 0.53
α2000 = 8h 58m 44s, δ2000 = -16° 07′ 58″
68 – 69 ly
Single star

HR 2007
Orion
mv = 5.97, mb = 6.61, B-V = 0.64
α2000 = 5h 48m 35s, δ2000 = -4° 05′ 41″
49.2 – 49.8 ly
Single star with exoplanet

The Eta Coronae Borealis system is noteworthy in that its two primary components are both G2V stars orbiting each other every 41.6 years.  The third component of this system is a distant infrared dwarf, spectral type L8V.

Two of these G2V stars host at least one exoplanet: HR 4523A in Centaurus and HR 2007 in Orion.

HR 4523A has a planet midway in mass between Uranus and Neptune orbiting every 122 days between 0.30 and 0.62 AU from the star (similar to orbital distance of the planet Mercury in our own solar system).  The other stellar component of this system. HR 4523B, is a distant M4V star currently orbiting about 211 AU from HR 4523A.

HR 2007, a single star like the Sun, has a planet about 78% more massive than Neptune, orbiting every 407 days, more or less.  If this planet were in our own solar system, it would range between the orbits of Venus and Mars, roughly.

Two Places, Same Meteor?

A good friend of mine, Paul Martsching, records meteor activity many nights a year for the American Meteor Society near Ames, Iowa, and has been doing so for many years.  On some of those nights, I am also recording meteor activity near Dodgeville, Wisconsin.  Is it possible for both of us to see the same meteor?

Paul’s observing location near Ames and my observing location near Dodgeville are separated by 180 miles.  Meteors burn up in the atmosphere at an altitude of about 50 miles.  Using a little simple trigonometry, we can find that the parallax angle between where Paul and I see the meteor is about 122°.  So, a meteor at either of our zeniths would be below the horizon at the other location.  If, on the other hand, Paul saw a bright meteor 29° above his NE horizon, I might be able to see the same meteor 29° above my SW horizon.

In general, if two observers are separated by a distance d in miles, then they will see the location of the meteor in the sky shifted by approximately s°, as given in the following equation:

This equation assumes that the curvature of the Earth is negligible, a reasonable assumption only when the two observers are relatively close to one another.

A more generalizable equation, taking into account the curvature of the Earth, though still assuming a spherical Earth is:

Plugging in the numbers, we get

We essentially get the same answer—a parallax angle of 122°.  In fact, using the small angle approximation tan x ≅ x for x << 1 (where tan x is in radians), the equation above simplifies to

If this looks a little familiar, it is.  Assuming the meteor burns up at an altitude of 50 miles, the equation immediately above becomes

which is our original equation!  So, for distances on the order of 200 miles or so (or less) we can completely ignore the curvature of the Earth.

Bringing Home Pieces of the Moon

The astronauts on Apollo 11, 12, 14, 15, 16, and 17 between 1969 and 1972 brought back a total of 840 lbs of moon rocks and soil.  Each successive Apollo mission brought back a larger amount of lunar material.

The Soviets brought back a total of 0.7 lbs of lunar soil through their robotic sample return missions Luna 16 (1970), Luna 20 (1972), and Luna 24 (1976).

So, excluding lunar meteorites that have befallen the Earth, a total of 840.7 lbs of lunar material has been delivered to research laboratories here on Earth.

It has been over 40 years since we have brought anything back from the lunar surface.  There are many interesting areas yet to be explored.  Why not send a series of robotic geologists to the Moon in advance of human missions? The success of the Spirit, Opportunity, and Curiosity rovers on Mars show us the exciting work that can be done at a fraction of the cost of human missions.  One enhancement would be the ability of the lunar robotic rovers to collect moon rocks and soil and return them to the mother ship for delivery to Earth.

But our 40+ year wait for additional lunar material may soon be over!

China plans to launch the Chang’e 5 lunar lander in November of this year.  It is expected to land in the Oceanus Procellarum (“Ocean of Storms”) region of the Moon, scoop up at least 4.4 lbs of lunar soil and rock—including some at least six feet below the surface!  The lunar haul will be launched into lunar orbit, where it will rendezvous with the sample return module that will bring it back to Earth.  After a high-speed entry into Earth’s atmosphere, the sample return module will rapidly decelerate, then gently parachute down to the Earth’s surface, presumably somewhere in China.

Chang’e 5 promises to be one of the most exciting and important space missions this year.  Stay tuned!

Two Predictions About Outdoor Lighting Technology

Here are my (ever hopeful) predictions about the future of outdoor lighting technology.

(1) Dusk-to-dawn lighting will soon become a thing of the past.

Ever see the irony that as outdoor lighting efficiency has greatly improved over the last several decades, we have moved from “light only when you need it” to “lights on all night long”?  An incandescent light, if operated less than 3 hours per night, will use less energy than even the most efficient light source operated dusk to dawn.  Yes, that’s right.  Three hours of incandescent light (which is horribly inefficient) each night throughout the year uses less energy than an LPS, HPS, Metal Halide, or LED source of comparable lumen output operated dusk-to-dawn.  Just think of the energy savings we could realize by using an efficient light source that is used only when it is needed!

Passive infrared (PIR) switches, which are rather prone to false triggering, will be replaced by image analysis software that will do a much better job of deciding when a light needs to be on and when it does not.

The HID (high intensity discharge) light sources in common use today such as HPS (high pressure sodium) and metal halide have two drawbacks.  They prematurely age if you frequently turn them on and off, and they take a while to reach full brightness after having been off for a while.  These drawbacks do not exist with efficient “instant on” sources such as LEDs, which are even dimmable.

These new technologies in lighting and control will make it both easy and affordable to have reliable light only when it is needed.

(2) Security lighting will soon be replaced by much better crime prevention technologies.

Soon, flooding a premises with light will be one of the WORST things you can do to deter and prevent crime.  As security systems improve and become more sophisticated and affordable, security lighting will only be needed when an intrusion is detected, and maybe not even then if you want the perpetrator to be detected without them knowing they have been detected.  Fixed visual recognition systems or even mobile peripheral devices (MPDs)—as Bill Gates likes to call “robots” to avoid all the anthropomorphic connotations—that operate with ambient light (visible, infrared, etc.) will soon obviate anything so primitive as security lighting. And, if the stationary or mobile sensing device is inactivated by a hostile (or non-hostile) event, its connection with the base station inside the home or business would be broken and appropriate action could be immediately taken.

As both lighting technology and lighting control technology improve, it is my hope that dusk-to-dawn lighting will be rendered obsolete.

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 last month, 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 half 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-)

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-)

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