By definition, pollution refers to any matter that is “out of place”. In other words, it is what happens when toxins, contaminants, and other harmful products are introduced into an environment, disrupting its normal patterns and functions. When it comes to our atmosphere, pollution refers to the introduction of chemicals, particulates, and biological matter that can be harmful to humans, plants and animals, and cause damage to the natural environment.
Whereas some causes of pollution are entirely natural – being the result of sudden changes in temperature, seasonal changes, or regular cycles – others are the result of human impact (i.e. anthropogenic, or man-made). More and more, the effects of air pollution on our planet, especially those that result from human activity, are of great concern to developers, planners and environmental organizations, given the long-term effect they can have.
There’s darkness out there in the cold corners of the solar system.
And we’re not talking about a Lovecraftian darkness, the kind that would summon Cthulhu himself. We’re talking of celestial bodies that are, well. So black, they make a Spinal Tap album cover blinding by comparison.
We recently came across the above true color comparison of Comet 67/P Churyumov-Gerasimenko adjusted for true reflectivity contrasted with other bodies in the solar system. 67/P is definitely in the “none more black” (to quote Nigel Tufnel) category as compared to, well, nearly everything.
Welcome to the wonderful world of albedo. Bob King wrote a great article last year discussing the albedo of Comet 67/P. The true albedo (or lack thereof) of 67/P as revealed by Rosetta’s NAVCAM continues to astound us. Are all comets this black close up? After all, we’re talking about those same brilliant celestial wonders that can sometimes be seen in the daytime, and are the crimson harbingers of regal change in The Game of Thrones, right?
There was also a great discussion of the dark realms of 67/P in a recent SETI Talk:
As with many things in the universe, it’s all a matter of perspective. If you live in the U.S. Northeast and are busy like we were earlier today digging yourself out from Snowmageddon 2015, then you were enjoying a planetary surface with a high albedo much more akin to Enceladus pictured above. Except, of course, you’d be shoveling methane and carbon dioxide-laced snow on the Saturnian moon… Ice, snow and cloud cover can make a world shinny white and highly reflective. Earthshine on the dark limb of the crescent Moon can even vary markedly depending on the amount of cloud and snow cover on the Earth that’s currently rotated moonward.
A brilliant Earthshine, or the ‘Old Moon in the New Moon’s arms’ from earlier last week. Photo by author.
To confound this, apparent magnitude over an extended object is diffused over its surface area, making the coma of a comet or a nebula appear fainter than it actually is. Engineers preparing for planetary encounters must account for changes in light conditions, or their cameras may just record… nothing.
For example, out by Pluto, Charon, and friends, the Sun is only 1/1600th as bright as seen here on sunny Earth. NASA’s New Horizons spacecraft will have to adjust for the low light levels accordingly during its historic flyby this July. On the plus side, Pluto seems to have a respectable albedo of 50% to 65%, and may well turn out to look like Neptune’s large moon, Triton.
Triton as imaged by Voyager 2: a dead ringer for Pluto? Credit: NASA/JPL.
And albedo has a role in heat absorption and reflection as well, in a phenomenon known as global dimming. The ivory snows of Enceladus have an albedo of over 95%, while gloomy Comet 67/P has an albedo of about 5%, less than that of flat black paint. A common practice here in Aroostook County Maine is to take fireplace ashes and scatter them across an icy driveway. What you’re doing is simply lowering the surface albedo and increasing the absorption of solar energy to help break up the snow and ice on a sunny day.
A high albedo snow cover blanketed New England earlier this week! Photo by author.
Ever manage to see Venus in the daytime? We like to point out the Cytherean world in the daytime sky to folks whenever possible, often using the nearby Moon as a guide. Most folks are amazed at how easy this daytime feat of visual athletics actually is, owing to the fact that the cloud tops of Venus actually have a higher albedo of 90%, versus the Moon’s murky 8 to 12%.
Venus (upper left) by daylight. Photo by author.
Apollo 12 command module pilot Richard Gordon remarked that astronauts Al Bean and Pete Conrad looked like they’d been “playing in a coal bin” on returning from the surface of the Moon. And in case you’re wondering, Apollo astronauts reported that moondust smelled like ‘burnt gunpowder’ once they’d unsuited.
The surface of the Moon closeup: darker than you think! Credit: Apollo 12/NASA.
Magnitude, global dimming and planetary albedo may even play a role in SETI as well, as we begin to image Earthlike exoplanets… will our first detection of ET be the glow of their cities on the nightside of their homeworld? Does light pollution pervade the cosmos?
And a grey cosmos awaits interstellar explorers as well. Forget Captain Kirk chasing Khan through a splashy, multi-hued nebula: most are of the light grey to faded green varieties close up. Through a telescope, most nebulae are devoid of color. It’s only when a long time exposure is completed that colors too faint to see with the naked eye emerge.
All strange thoughts to consider as we scout out the dark corners of the solar system. Will the Philae lander reawaken as perihelion for Comet 67/P approaches on August 13th, 2015? Will astronauts someday have to navigate over the dark surface of a comet?
I can’t help but think as I look at the duck-like structure of 67/P that one day, those two great lobes will probably separate in a grand outburst of activity. Heck, Comet 17P/Holmes is undergoing just such an outburst now — one of the best it has generated since 2007 — though it’s still below +10th magnitude. How I’d love to get a look at Comet 17P/Holmes up close, and see just what’s going on!
This 4th of July weekend brings us one more reason to celebrate. On July 5th at approximately 11:00 AM EDT/15:00 UT, our fair planet Earth reaches aphelion, or its farthest point from the Sun at 1.0167 Astronomical Units (A.U.s) or 152,096,000 kilometres distant.
Though it may not seem it to northern hemisphere residents sizzling in the summer heat, we’re currently 3.3% farther from the Sun than our 147,098,290 kilometre (0.9833 A.U.) approach made in early January.
We thought it would be a fun project to capture this change. A common cry heard from denier circles as to scientific facts is “yeah, but have you ever SEEN it?” and in the case of the variation in distance between the Sun and the Earth from aphelion to perihelion, we can report that we have!
We typically observe the Sun in white light and hydrogen alpha using a standard rig and a Coronado Personal Solar Telescope on every clear day. We have two filtered rigs for white light- a glass Orion filter for our 8-inch Schmidt-Cassegrain, and a homemade Baader solar filter for our DSLR. We prefer the DSLR rig for ease of deployment. We’ve described in a previous post how to make a safe and effective solar observing rig using Baader solar film.
Our primary solar imaging rig. A Nikon D60 DSLR with a 400mm lens + a 2x teleconverter and Baader solar filter. Very easy to employ!
We’ve been imaging the Sun daily for a few years as part of our effort to make a home-brewed “solar rotation and activity movie” of the entire solar cycle. We recently realized that we’ve imaged Sol very near aphelion and perihelion on previous years with this same fixed rig, and decided to check and see if we caught the apparent size variation of our nearest star. And sure enough, comparing the sizes of the two disks revealed a tiny but consistent variation.
It’s a common misconception that the seasons are due to our distance from the Sun. The insolation due to the 23.4° tilt of the rotational axis of the Earth is the dominant driving factor behind the seasons. (Don’t they still teach this in grade school? You’d be surprised at the things I’ve heard!) In the current epoch, a January perihelion and a July aphelion results in milder climatic summers in the northern hemisphere and more severe summers in the southern. The current difference in solar isolation between hemispheres due to eccentricity of Earth’s orbit is 6.8%.
The orbit of the Earth also currently has one of the lowest eccentricities (how far it deviates for circular) of the planets at 0.0167, or 1.67%. Only Neptune (1%) and Venus (0.68%) are “more circular.”
The orbital eccentricity of the Earth also oscillates over a 413,000 year period between 5.8% (about the same as Saturn) down to 0.5%. We’re currently at the low end of the scale, just below the mean value of 2.8%.
Variation in eccentricity is also coupled with other factors, such as the change in axial obliquity the precession of the line of apsides and the equinoxes to result in what are known as Milankovitch cycles. These variations in extremes play a role in the riddle of climate over hundreds of thousands of years. Climate change deniers like to point out that there are large natural cycles in the records, and they’re right – but in the wrong direction. Note that looking solely at variations in the climate due to Milankovitch cycles, we should be in a cooling trend right now. Against this backdrop, the signal of anthropogenic climate forcing and global dimming of albedo (which also masks warming via cloud cover and reflectivity) becomes even more ominous.
Aphelion can presently fall between July 2nd at 20:00 UT (as it did last in 1960) and July 7th at 00:00 UT as it last did on 2007. The seemingly random variation is due to the position of the Earth with respect to the barycenter of the Earth-Moon system near the time of aphelion. The once every four year reset of the leap year (with the exception of the year 2000!) also plays a lesser role.
Perihelion and aphelion vs the solstices and equinoxes, an exaggerated view. (Wikimedia Commons image under a 3.0 Unported Attribution-Share Alike license. Author Gothika/Doudoudou).
I love observing the Sun any time of year, as its face is constantly changing from day-to-day. There’s also no worrying about light pollution in the solar observing world, though we’ve noticed turbulence aloft (in the form of bad seeing) is an issue later in the day, especially in the summertime. The rotational axis of the Sun is also tipped by about 7.25° relative to the ecliptic, and will present its north pole at maximum tilt towards us on September 8th. And yes, it does seem strange to think in terms of “the north pole of the Sun…”
We’re also approaching the solar maximum through the 2013-2014 time frame, another reason to break out those solar scopes. This current Solar Cycle #24 has been off to a sputtering start, with the Sun active one week, and quiet the next. The last 2009 minimum was the quietest in a century, and there’s speculation that Cycle #25 may be missing all together.
And yes, the Moon also varies in its apparent size throughout its orbit as well, as hyped during last month’s perigee or Super Moon. Keep those posts handy- we’ve got one more Super Moon to endure this month on July 22nd. The New Moon on July 8th at 7:15UT/3:15 AM EDT will occur just 30 hours after apogee, and will hence be the “smallest New Moon” of 2013, with a lot less fanfare. Observers worldwide also have a shot at catching the slender crescent Moon on the evening of July 9th. This lunation and the sighting of the crescent Moon also marks the start of the month of Ramadan on the Muslim calendar.
Be sure to observe the aphelion Sun (with proper protection of course!) It would be uber-cool to see a stitched together animation of the Sun “growing & shrinking” from aphelion to perihelion and back. We could also use a hip Internet-ready meme for the perihelion & aphelion Sun- perhaps a “MiniSol?” A recent pun from Dr Marco Langbroek laid claim to the moniker of “#SuperSun;” in time for next January’s perihelion;
