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.
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.
One hundred and twenty five years ago this month, on January 1, 1892, two Germans, astronomer & physicist Martin Brendel (1862-1939) and geographer & meteorologist Otto Baschin (1865-1933), arrived at Alta fjord near Bossekop in northern Norway to study the Northern Lights and conduct magnetic field measurements. Their latitude was just shy of 70° N. Brendel began photographing the aurora the next day, and his first extant photograph (the first ever) was taken on January 5, 1892.
Edward Emerson Barnard (1857-1923), incidentally, was to establish his reputation as an extraordinarily gifted astrophotographer later that same year when he began taking photographs of comets, clusters, nebulae (including galaxies), and the Milky Way using the 6-inch Crocker astrographic camera at the Lick Observatory.
Catchers of the Light: A History of Astrophotography by Stefan Hughes
A small number of asteroids are currently in a temporary 1:1 orbital resonance with the Earth in their orbit around the Sun. Unlike the Moon, which is in a stable orbit around the Earth, these much tinier “co-orbital” objects are “just passin’ through.”
54509 YORP (2000 PH5)
This tiny asteroid, perhaps 492 × 420 × 305 feet across, is a rapid rotator, turning around once every 12m10s. It is named after the YORP effect, as it provided the first observational evidence of that effect speeding up its spin rate. It’s day will be half as long in only 600,000 years, and it may eventually speed up to one rotation every 20 seconds!
Wiki JPL Orrery
This near-Earth object has an orbit that is very similar to the Earth’s, and even more circular, though it is inclined a full 10.7° to the ecliptic. This asteroid is a good candidate for an automated sample-return mission and then human exploration because it is relatively close to the Earth and the amount of energy needed to visit 2002 AA29 and return to Earth is relatively small.
Wiki JPL Orrery
This extremely tiny object (just 7 to 10 feet across) spins more rapidly than any other object on our list: once every 2m45s! It may even be an old rocket booster from the Apollo era, but recent evidence indicates it is a bona fide space rock. It is currently leading the Earth in a very similar orbit.
Wiki JPL Orrery
419624 (2010 SO16)
This asteroid was discovered using an infrared space telescope (WISE) and is in an unusually stable orbit that will change little during the next several hundred thousand years. It is currently trailing the Earth.
Wiki JPL Orrery
469219 (2016 HO3)
Currently, a quasi-satellite of the Earth. Always remains within 38 to 100 lunar distances from the Earth as it orbits the Sun. Leads, then follows, then leads again. Quite a do-si-do!
Wiki JPL Orrery
The orrery videos for each asteroid were generated using the Jet Propulsion Laboratory’s incredible Orbit Diagram Java applet on their Small Body Database Browser web site (https://ssd.jpl.nasa.gov/sbdb.cgi), and captured using the equally incredible ScreenFlow software from Telestream (https://www.telestream.net/screenflow/). Kudos to both organizations!
As Dodgeville (and many other towns and cities) are planning to replace their streetlights with LED luminaires, it is imperative that we use LEDs with a CCT (correlated color temperature) of 3000 K or less (Jin et al. 2015). This is a “warm” white light (similar to incandescent) rather than the “cold” blue-rich light often seen with LEDs. Outdoor LED luminaires often come in at least three “flavors”: 3000K, 4000K, and 5000K. For example, American Electric Lighting’s Autobahn Series. 5000K luminaires provide the bluest light, and should be avoided at all costs. Of these three, 3000K would be best, and if 2700K is offered, use that.
Why does this matter? On June 14, 2016, the American Medical Association issued guidance on this subject.
Incidentally, for your residential lighting needs, a good local source for LED bulbs that are not blue-rich is Madison Lighting. They have many LED bulbs in both 3000 K and 2700 K. I use 2700K bulbs exclusively in my home, and the warm white light they provide is an excellent replacement for incandescent and compact fluorescent bulbs. Never purchase LED lighting without knowing the color temperature of the lights.
If you’re skeptical that the color temperature of LEDs is an important issue, I suggest you purchase a 2700K bulb and a 4000K or 5000K bulb with the same output lumens and compare them in your home. I believe that you will much prefer the 2700K lighting. If 2700K lighting is best for your home, then why should it not be best for outdoor lighting as well?
Besides, most streetlighting is currently high pressure sodium (HPS), which is inherently non-blue-rich. You will find that 2700K LED lights offers better color rendering than HPS without the need to go to even bluer lights.
If you have ever been irritated at night by an oncoming vehicle with those awful “blue” headlights, you’ve experienced firsthand why blue-rich light in our nighttime environment must be minimized.
Why are 4000K and 5000K LED lights so prevalent? They are easier and cheaper to manufacture, but with increased demand of 2700K and 3000K LED lights, economies of scale will reduce their cost, which today are generally slightly higher than blue-rich LEDs.
Now, a bit more about why blue light at night can be detrimental to human health, and the primary reason why the AMA issued a guidance on this subject.
In addition to image-forming rods and cones, there exist non-image-forming retinal cells in the human eye called intrinsically photosensitive retinal ganglion cells (ipRGCs) that help regulate our circadian rhythms. Studies have shown that blue light is far more disruptive to our circadian rhythms than redder light (Lockley et al. 2003).
Now, on to the environment. Using a clever technique that compared sky brightness at several locations on several nights both with and without snow cover, Fabio Falchi (Falchi 2011) determined that at least 60% of light going up into the night sky is direct waste lighting, and 40% or less is reflected light. This is as good an argument as any that we still have a long way to go towards using only full-cutoff luminaires that do not produce any direct uplight. Blue light scatters much more in the night sky than red light, and this is due to Rayleigh scattering which tells us that the amount of scattering is proportional to the inverse of the wavelength of light to the fourth power, σs ∝ 1 / λ4. This also explains why the daytime sky is blue.
Bluer wavelengths of light thus increase artificial sky glow to a much greater extent than redder wavelengths do. Not only is an increase in blue light bad for astronomy, but its impact on the natural world is likely to be adverse as well.
Falchi recommends a total ban of wavelengths shorter than 540 nm for nighttime lighting, both outdoor and indoor. He goes on to say that, at the very least, no more light shortward of 540 nm should be allowed than that currently emitted by high pressure sodium lamps, lumen for lumen.
Falchi, F. 2011, MNRAS, 412, 33
Falchi, F. 2016, The World Atlas of Light Pollution, p. 44
Jin, H., Jin, S., Chen, L., et al. 2015, IEEE Photonics Journal. 7(6), 1-9
Lockley, S. W., et al. 2003, J Clin Endocrinol Metab. 88(9), 4502–5
The space between stars is not a perfect vacuum. It contains gas molecules and dust grains, although they are few and far between by any terrestrial standard. In the presence of a magnetic field, many types of interstellar dust grains line up in a way that is reminiscent of iron filings near a bar magnet. When light from a star passes through a region of space with magnetically-aligned dust grains (though in this case the short axis of the dust grains aligns with the local magnetic field), light with the electric field vector perpendicular to the long axis of the grains is less likely to be absorbed by the grains than light whose electric field vector is parallel to the long axis of the grains. This causes the light passing through such regions of space to become slightly polarized, and the polarization of starlight is something we can measure easily here on Earth. In this way, the strength and orientation of invisible interstellar or circumstellar magnetic fields can be determined at a distance.
Various astrophysical processes result in polarized electromagnetic radiation. The differential absorption already mentioned polarizes the light from all stars to one degree or another. Only the Sun—which is vastly nearer—offers us almost completely unpolarized light. Scattering of light off of interstellar clouds and planetary surfaces also results in polarization. Finally, both synchrotron and cyclotron emission produce a characteristic polarization.
The polarization of starlight can be measured by the use of a polarimeter attached to the telescope. Unlike standard photometry, polarization is simpler to measure with ground-based telescopes because the measurements are relative rather than absolute and, under normal circumstances, the Earth’s atmosphere does not affect the polarization state of incoming light. Care must be taken, however, to ensure that the telescope itself does not create instrumental polarization due to oblique reflections. Placing the polarimeter at the unfolded Cassegrain focus is one desirable configuration (Hough 2006).
Hough, J. 2006, A&G, 47, 3.31
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
On Tuesday, December 19, 2006, I witnessed a delightful event: the Space Shuttle Discovery and the International Space Station traveling together through the western sky, only about 1° apart.
Around 6:34 p.m., I spotted a -1 magnitude International Space Station (ISS) traveling NE above the western horizon. It quickly became apparent that there was a +1 magnitude point of light moving right along with the ISS, leading it by about one degree. It was the Space Shuttle Discovery, which had undocked from the ISS just 2h25m earlier (4:09 p.m.)!
I quickly surmised that Discovery must have fired retrorockets to put some distance between it and the ISS by lowering Discovery‘s altitude. Since Discovery was at a lower altitude, it had been orbiting faster, which is why it was leading the ISS by about a degree. As the pair approached the constellation Lyra, further evidence of Discovery‘s lower altitude occurred when it disappeared into the shadow of the Earth several degrees further west of where the ISS disappeared a few seconds later.
Did you know that a few percent of all stars are traveling alone through intergalactic space, no longer a part of any galaxy? Gravitational interactions between stars or between stars and black holes can occasionally accelerate a star to galactic escape velocity so that it is thrown (eventually) into intergalactic space. When the star first enters intergalactic space, the view of your home galaxy would be pretty remarkable, but eventually (eons later, of course) there would be very few naked eye objects in your night sky. Just moons and planets, meteors, aurora, comets, the zodiacal light, and maybe a galaxy or two. Anything else would require a telescope. And an observer, of course.
The first evidence for intergalactic stars came from the detection of diffuse light between galaxies (Zwicky 1952). Much later, intergalactic planetary nebulae were detected in the Fornax galaxy cluster (Theuns & Warren 1997). More recently, intergalactic red giant stars have been detected in the Virgo galaxy cluster using the Hubble Space Telescope (Ferguson et al. 1998).
The Fornax cluster lies about 62 million light years distant, and the Virgo cluster 54 million light years distant. Have any intergalactic stars been detected near our Milky Way galaxy? Brown et al. (2005) discovered the first hypervelocity star, SDSS J090745.0+024507, a 20th-magnitude star in the constellation Hydra. Though it is just 160,000 light years from the center of our galaxy, it is moving away from the Galactic center at an astonishing radial velocity of 709 km/s. Even though this one-dimensional radial velocity1 is only a lower limit to the star’s true 3D space motion, it is far and away fast enough to escape our Milky Way galaxy altogether. Gaia will probably be able to measure this runaway star’s proper motion in right ascension and declination, thus allowing a determination of the true space velocity of SDSS J090745.0+024507 relative to the Galactic center.
Several more hypervelocity stars have been discovered since 2005. One of them, US 708, a 19th-magnitude white dwarf in Ursa Major, is exiting our galaxy at a velocity of at least 1200 km/s! This makes it the fastest on record (Geier et al. 2015).
1The observed one-dimensional radial velocity as seen from Earth is corrected for the Earth’s rotation and motion around the Sun, and the Sun’s motion around the center of the Milky Way galaxy to determine the galactocentric radial velocity.
Brown, W. R., Geller, M. J., Kenyon, S. J., Kurtz, M. J. 2005, ApJ, 622, L33
Ferguson, H. C., Tanvir, N. R., & von Hippel, T. 1998, Nature, 391, 461
Geier, S., Fürst, F., Ziegerer, E., et al. 2015a, Science, 347, 1126
Theuns T., Warren S. J., 1997, MNRAS, 284, 11
Zwicky F., 1952, PASP, 64, 242
It is hard to believe that Carl Sagan has been gone now for 20 years. In fact, he died on this day in 1996 of myelodysplastic syndrome at the age of 62. He was one of the 20th century’s truly great science popularizers. In addition to writing or co-writing fifteen books, his 1980 PBS television series Cosmos remains the gold standard against which all other astronomy documentaries will be judged.
Here is a listing of Carl Sagan’s books published during his lifetime:
- Intelligent Life in the Universe (1966; revised and expanded edition of Iosif Shklovsky’s 1962 book of the same name)
- Planets (1966; one of the LIFE Science Library series)
- The Cosmic Connection (1973)
- Communication with Extraterrestrial Intelligence: CETI (1973; Carl Sagan, editor)
- Other Worlds (1975)
- The Dragons of Eden: Speculations on the Evolution of Human Intelligence (1977)
- Murmurs of Earth: The Voyager Interstellar Record (1978; with others)
- Broca’s Brain: Reflections on the Romance of Science (1979)
- Cosmos (1980)
- Contact (1985)
- Comet (1985; with Ann Druyan)
- Shadows of Forgotten Ancestors: A Search for Who We Are (1993; with Ann Druyan)
- Pale Blue Dot: A Vision of the Human Future in Space (1994)
- The Demon-Haunted World: Science as a Candle in the Dark (1995)
- Billions and Billions: Thoughts on Life and Death at the Brink of the Millennium (written 1996, published posthumously in 1997)
Carl Sagan’s final interview was with Charlie Rose on May 27, 1996, less than seven months before his death. You can see it here.
Carl’s daughter, Sasha Sagan, wrote a loving and thoughtful essay in 2014, the 80th anniversary year of his birth.
Here, now, are just a few of Carl Sagan’s most memorable quotes.