See the faint concentration of light toward the center of this image? Thats the elusive gegenschein aka the counterglow a diffuse spot visible in the darkest of skies, centered at the point directly opposite the sun. Sunlight reflecting on interplanetary dust causes it. Image via Project Nightflight.
Article and most photos by Erwin Matys and Karoline Mrazek of Project Nightflight, whose mission is to promote the conservation of the starry sky as environmental resource.
The suns counterglow or gegenschein is kind of a stargazers legend. Every amateur astronomer has heard about it, only a few of them have actually seen it, and even fewer were lucky enough to capture an image of this dim and ghostlike apparition. As a fellow observer put it:
The gegenschein is certainly not a GoTo object.
Matter of fact, it isnt an object at all. But lets start from the beginning.
What exactly is the gegenschein?
It is widely known that the space between the planets isnt empty. The plane of the solar system is filled with an enormous disk of small dust particles with sizes ranging from less than 1/1000 millimeter up to 1 millimeter. It is less commonly known that this interplanetary dust cloud is a highly dynamic structure. In contrast to conventional wisdom, it is not an aeon-old leftover from the solar systems formation. That primordial dust is long gone. Todays interplanetary dust is in an astronomical sense of speaking very young, only millions of years old. Most of the particles originate from quite recent incidents, like asteroid collisions.
This is not the gegenschein. The picture shows the zodiacal light, which is closely related to the gegenschein. Here imaged from a rural site, the zodiacal light is a cone of light extending from the sun along the ecliptic, visible after dusk and before dawn. The gegenschein stems from the same dust cloud, but is always opposite the sun, for example, highest in the sky at midnight. The gegenschein is much harder to detect or photograph than the zodiacal light. Image via Project Nightflight.
Exposed to various forces, the particles do not remain in stationary orbits but inhabit the disk in ever-changing motion. The smallest particles (less than 1/1000 millimeter) are swiftly blown out of the solar system by the solar wind. The larger particles dont survive very long either. They tend to collide with larger bodies or slowly spiral into the inner solar system where they fall into the sun.
Nevertheless, the supply of interplanetary dust particles is constantly replenished by the above-mentioned asteroid collisions and the erosion of comets. So, the interplanetary dust cloud of the solar system is not a static formation but a dynamic structure consisting of quite young components.
For an observer on Earth, this dynamic dust cloud is mainly visible in the form of the zodiacal light. After dusk and before dawn it extends as a cone of light from the sun along the ecliptic path. Often called the false dawn, the zodiacal light is quite bright and can be seen from any observing site that doesnt suffer from severe light pollution, especially when the ecliptic is high in the sky. For Northern Hemisphere observers, this is the case in the evening sky during spring and in the morning sky during autumn.
The morning and evening zodiacal light are the two areas of the interplanetary dust disk where sunlight gets forward scattered to earth, resulting in the bright silvery light cones. The interplanetary dust disk is also visible along the rest of the ecliptic path, where it is called the zodiacal band. Unlike the zodiacal light, however, these sections are very hard to detect since they have an extremely low surface brightness. But at the point directly opposite the sun the geometry again works in our favor and enhances the visibility of the interplanetary dust. This is the area of the gegenschein.
But why does the gegenschein glow brighter than the rest of the zodiacal band? Looking at the interplanetary dust disk from Earth, the section at the antisolar point is illuminated from directly behind our heads. This results in an increase in brightness that is called the opposition effect. The opposition effect is a frequent phenomenon for solar system bodies. For example, the moon shows a significant peak in brightness around full moon, i.e. at its opposition. Another example are Saturns rings in the days around opposition, when they brighten drastically.
The main reason for this brightening during opposition is shadow hiding, meaning that all particles are fully illuminated. This opposition effect can even be experienced here on Earth during daylight if you are outdoors and look at the ground in front of you. If the ground is sufficiently coarse, you will see a brightening around the shadow of your head. To document this phenomenon, we made the photo below, which shows the opposition effect on volcanic gravel.
This is like the gegenschein. For this photo, a small action cam was positioned on a tripod looking down on volcanic gravel on La Palma island. The insert shows some of the gravels with a size of several millimeters and their rough surface structure. Around the cameras shadow the opposition effect results in an obvious brightening. The glow pictured here is similar to the glow of the gegenschein on the night sky. Image via Project Nightflight.
Interestingly, the brightening around the cameras shadow had an apparent diameter of 10 degrees, which is about the same size as the diameter of the gegenschein in the night sky. Next time you walk on a rough surface in sunlight, give it a try and look for this terrestrial version of the counterglow.
To sum it up, the gegenschein is not an object. It is a play of light on the solar systems interplanetary dust disk. The section of the dust at the antisolar point is squarely illuminated, which results in a brightness enhancement due to shadow hiding. This so-called opposition effect can be encountered at many occasions, but with the gegenschein it makes its most ghostlike and outlandish appearance. This is probably the reason why it is such a prized trophy among amateur astronomers.
How we captured the gegenschein
From our observing and photography sessions at dark-sky sites, the gegenschein was a familiar companion. Out under the stars, sooner or later one of us would mention: Mmh, the gegenschein is quite obvious tonight, followed by the typical reply: Yep, bright and shiny! This short dialogue would indicate that our eyes had reached full dark adaptation and our night vision was at maximum capacity. We often talked about making an image of our good old dark-sky companion, the gegenschein. But for some reason it never came to it. Other projects were in the way, on other occasions the position in the sky was not right, or the atmospheres transparency was just not good enough.
But on the night from October 30 to 31, 2019, we finally did give it a go. On October 30 we were already several days on an imaging excursion on La Palma island. During daytime the annoying Calima weather pattern had finally stopped and skies were again as deep blue and transparent as they can be on this beautiful stars island. Accommodated in a solitary finca far away from inhabited areas and about 800 meters (2,600 feet) above sea level we had the perfect conditions for photographing the gegenschein. Around midnight, when the counterglow culminates, our instruments registered a sky brightness of 21.4 magnitudes per square arcsecond in the zenith. This is so dark that Sirius noticeably brightened the landscape when it rose behind a mountain ridge later that night. The gegenschein itself was pretty obvious to the unaided eye. Below the constellation Aries, directly on the ecliptic path, a distinct glow about 10 degrees across marked the antisolar point. The very faint zodiacal band crossing the whole sky was visible too, almost looking like an artificial marking of the ecliptic. Under these conditions, shooting the gegenschein was an easy task.
For those interested in the technical details: we made 23 unfiltered exposures of 4 minutes with a 16mm lens @f/5.6 on a Baader-modified 1100D Canon DSLR. For sky tracking we used a purely mechanical device, the Mini Track LX2. This innovative device is a frequent companion on our imaging excursions. For those interested in more details, we provide a thorough review of the Mini Track LX2 for download on our website. The total weight of our imaging gear was less than 3 kilograms (6.6 pounds), including camera and tripod. The 23 individual exposures of the gegenschein were later calibrated, registered and stacked with DeepSkyStacker. Processing of the final image was done in Photoshop, where we applied substantial contrast enhancement.
How you can see the gegenschein
If you never encountered the gegenschein yourself, dont expect it to be as prominent as in the photograph at the top of this post. We applied heavy contrast enhancement to the image to make the counterglow, its shape, and its size better visible. To our human eyes, even under the best conditions the gegenschein is an elusive and dim glow. On several occasions we compared the visual brightness of the counterglow and sections of the Milky Way. We always found it to be about as bright as the dimmest parts of the winter Milky Way. The stretch of the Milky Way that approximates the counterglows brightness best is the section between the star Mirfak and the asterism called the Kids next to Capella. That is, very dim. Any bright object in its vicinity (e.g. Jupiter or Mars at opposition) makes it hard to observe.
Aside from being slightly brighter in the middle and fading out uniformly in all directions the gegenschein shows no structure at all. It has a roundish form, circular or elliptical, measuring approximately 10 degrees across. That is about the size of a fist stretched out at arms length.
Drawing of the gegenschein as seen on November 23, 2019, from a dark-sky observing site in Lower Austria. On this night the antisolar point was between the 2 star clusters of the Hyades and Pleiades. The drawing gives a realistic impression of what to expect when looking for the counterglow. Image via Project Nightflight.
If you want to hunt down the elusive gegenschein yourself, the tips below might come handy:
First of all, check the position: Before you begin searching for the gegenschein in the night sky you should check its current position. The map below helps you to determine where to look for it. It also shows you whether the counterglow is detectable at all, because its visibility highly depends on its position against the starry background. In June, July and December it is completely invisible, because it is in front of the Milky Way. Other obstacles can be the bright planets Mars and Jupiter. When one of them is near opposition, it is in the same region of the sky as the counterglow and may outshine it. Some planetarium programs or stargazing apps might also be useful tools to determine the position of the gegenschein. If the software provides an option to display the center of the earths shadow this will show you the current antisolar point.
View larger. | This map gives you an idea where to look for the gegenschein in front of the starry background. The scale at the bottom indicates the constellations that culminate at your local midnight on a given date. This also approximates where you can find the gegenschein on the dotted line of the ecliptic. For example, at the end of March it will glow in the constellation Virgo. Image via Project Nightflight.
Look for it at maximum height above the horizon: The gegenschein is always highest above the horizon around local midnight, so that is the best time of the night to give an observation a try. There should also be no moon above the horizon. But thats not all. Like the sun itself, the antisolar point too has different culmination heights during the course of the year. For observers in the Northern Hemisphere the antisolar point has its highest culmination at the winter solstice around December 21. Sadly, during December the gegenschein is in front of the Winter Milky Way and therefore invisible. So, the best months to see the gegenschein from northern latitudes are November and January. For Southern Hemisphere observers, the largest culmination height of the antisolar point happens at summer solstice around June 21. For southerners too, the Milky Way is in the way. Again, one should look before or after the solstice, in this case during May or August.
View larger. | Only if all of the conditions listed in this diagram are true, you have a valid chance of actually detecting the gegenschein. Image via Project Nightflight.
Try from the best observing location you have access to: The gegenschein cannot be seen from light-polluted sites. Even moderate light pollution diminishes the counterglows contrast way too much. Based on our observations, the absolute minimum to detect the gegenschein is a sky brightness of 21.0 mag/arcsecond2 in the zenith. But this applies only if you are already quite familiar with the gegenschein and know exactly what and where to look for. For first-time observers we recommend a site with a sky brightness of 21.2 mag/arcsecond2 in the zenith or better. These conditions can only be found far away from cities or other inhabited areas. Observing sites in the mountains especially qualify because of the reduced air mass at higher altitudes above sea level.
Wait for favorable weather conditions: Even if you observe from a dark-sky location, the average clear sky might not be good enough for seeing the gegenschein. What you really want is a night sky with exceptionally high transparency. This will only be the case if the air mass above your observing site is as dry as possible. To get an idea of the current situation at your location, you can use a website that provides weather forecasts for astronomical observations (e.g. 7timer.info). Sites like this display data about humidity at all tropospheric layers. Another condition for high transparency would be that the air is clean, i.e. free of dust. At mid-latitudes, the necessary conditions of dry and clean air often can be found after the passage of a cold front or a trough.
Expand your observing skills: Given you have good eyesight in the dark and you already have some observing experience, there are several observing techniques you need to master for the gegenschein. First of all, you need to learn how to become truly dark adapted. This might put your patience to a test, since your eyes need at least 3/4 of an hour to fully adjust to the dark. So, no smartphones or other handheld devices during this time. You should only use very dim red lights if you dont want to ruin your night vision. A second technique you might need for seeing the gegenschein is averted vision. With averted vision, you do not look directly at an object but a little off to the side, while continuing to concentrate on the object. This way you are using peripheral vision which is more sensitive to low light levels than the center of the eye. Some observers report that this technique makes a big difference for them and it might help you too. In any case, hunting down the gegenschein will improve your observing skills a lot. Besides, it makes a highly interesting project and sooner or later you too will be able to put that prized stargazers trophy on your shelf.
About Project Nightflight: Our mission is the starry sky. We internationally promote the conservation of the starry sky as environmental resource.To support this goal, we work together closely with our sponsors and international news media. We provide news portals, nature related websites, books, magazines and newspapers with high resolution images of the unspoiled night sky, catching stories about the magic of the starry sky, useful tips for stargazing and astrophotography and informative articles on light pollution prevention. With our images and stories, which are frequently published by major print and online news media, we raise awareness for the need to keep light pollution at bay. Our team is based in Vienna, Austria, and consists of experienced, world-wide active astrophotographers who work on a volunteer basis. Most of our active members are marketing or communication professionals. If you want to learn more about our organization please download the Project Nightflight profile.
Bottom line: The gegenschein isnt an object. Its a play of light on the solar systems interplanetary dust disk. The gegenschein lies at the antisolar point, the point exactly opposite the sun. At this point, sunlight squarely illuminates the dust that moves between the planets. In this way, the gegenschein is related to the zodiacal light, which also stems from sunlight reflecting from dust between the planets. But the gegenschein is much more elusive than the zodiacal light. Its a diffuse patch in the night sky, which moves directly opposite the sun.
Via Project Nightflight
Read the original:
- The Way of the Shogun | Travel - Smithsonian Magazine - July 12th, 2020
- Astronomers discover South Pole Wall, a gigantic structure stretching 1.4 billion light-years across - Livescience.com - July 11th, 2020
- Astronomers think they can find the Sun's lost siblings - Astronomy Magazine - July 11th, 2020
- Beyond the twilight zone: Living on a two-faced world - Astronomy Magazine - July 11th, 2020
- 4 mysterious objects spotted in deep space are unlike anything ever seen - Livescience.com - July 11th, 2020
- Astronomers Spot Strange Space Object, Have No Idea What It Is - Popular Mechanics - July 11th, 2020
- NYU Astronomer Searches for Life, and Meaning, on Other Planets - NYU News - July 11th, 2020
- Look Up! Astronomer Says July Skies Are Brimming With Planets, Constellations - Here And Now - July 11th, 2020
- 'Clyde's Spot,' a new storm on Jupiter, discovered by amateur astronomer (photos) - Space.com - July 11th, 2020
- 3 takes on dealing with uncertainty Harvard Gazette - Harvard Gazette - July 11th, 2020
- Astronomers Find Stellar Stream of Extragalactic Origin in the Vicinity of the Sun | Astronomy - Sci-News.com - July 11th, 2020
- Astronomy - Wikipedia - July 5th, 2020
- astronomy | Definition & Facts | Britannica - July 5th, 2020
- What is Astronomy? Definition & History | Space - July 5th, 2020
- Astronomy for Beginners | Night Sky Facts, FAQs ... - July 5th, 2020
- Astronomers Have Located The Centre of The Solar System to Within 100 Metres - ScienceAlert - July 5th, 2020
- First planetary core discovered, astronomers announce - USA TODAY - July 5th, 2020
- How Elon Musk's Starlink satellites may have stifled the best chance to find Planet 9 - CNN - July 5th, 2020
- Astronomers Might Have Seen a Star Just Disappear. Turning Straight to a Black Hole Without a Supernova - Universe Today - July 5th, 2020
- Up close and personal: 10 years in the life of the Sun - Astronomy Magazine - July 5th, 2020
- Exoplanet the Size of Neptune Discovered by Astronomers About 32 Light-Years From Earth - SciTechDaily - July 5th, 2020
- Social Divisions Drive Astronomical COVID-19 Rate In Chile : Goats and Soda - NPR - July 5th, 2020
- How stargazing could save your life | 1843 - The Economist 1843 - July 5th, 2020
- The Sky This Week from June 19 to 26 - Astronomy Magazine - June 22nd, 2020
- Five Jansky Fellows Look to the Future of Radio Astronomy - Newswise - June 22nd, 2020
- Enjoy the sky's great globular - Astronomy Magazine - June 22nd, 2020
- An unprecedented look at the atmosphere of the supergiant star Antares - Astronomy Magazine - June 22nd, 2020
- Explore the world of galaxies - Astronomy Magazine - June 22nd, 2020
- Infinity and Beyon: Episode 1 The Big Bang - Astronomy Magazine - June 22nd, 2020
- Astronomers Estimate There Are 6 Billion Earth-Like Planets in the Milky Way - Universe Today - June 22nd, 2020
- CHIME study involving U of T astronomers finds fast radio bursts repeat 'on the time scale of weeks' - News@UofT - June 22nd, 2020
- 1981: Barbara Williams becomes the first Black woman to get a PhD - Astrobites - June 22nd, 2020
- Astronomers Detect a Fascinating Green Glow in The Atmosphere of Mars - ScienceAlert - June 22nd, 2020
- UI professors mapping the way for the future of plasma physics and fusion energy research - UI The Daily Iowan - June 22nd, 2020
- End of the world: Astronomers break silence over June 21 doomsday - Express.co.uk - June 22nd, 2020
- For Astronomy Buff Sushant Singh Rajput, A Tribute From France's International Space University - NDTV - June 22nd, 2020
- UArizona Offers Free and Discounted Programs to Meet Needs of New Learners - UANews - June 22nd, 2020
- Sun-Like Star Kepler-160 Has Super-Earth in Habitable Zone | Astronomy - Sci-News.com - June 9th, 2020
- Half the matter in the cosmos was missing, but astronomers found it h - Astronomy Magazine - June 9th, 2020
- What has the Juno spacecraft taught us about Jupiter? - Astronomy Magazine - June 9th, 2020
- The Sky This Week from June 5 to 12 - Astronomy Magazine - June 9th, 2020
- The History and Future of Telescopes on the Moon - Astronomy Magazine - June 9th, 2020
- The 1st stars in the universe formed earlier than thought - Space.com - June 9th, 2020
- Astronomers have found a planet like Earth orbiting a star like the sun - MIT Technology Review - June 9th, 2020
- How 'Mars Undergound' sparked a return to the Red Planet - Astronomy Magazine - June 9th, 2020
- Astronomers capture rare cosmic 'Jekyll and Hyde' behaviour in double star system 19,000 light-years away - Folio - University of Alberta - June 9th, 2020
- First Optical Measurements of Milky Ways Fermi Bubbles Probe Their Origin - ERAU News - June 9th, 2020
- A rendezvous between Jupiter, Saturn and the moon lit up the night skies this week - Chron - June 9th, 2020
- Astronomers Pinpoint The Origin of Huge Gas Bubbles Flowing Out of The Milky Way - ScienceAlert - June 9th, 2020
- It's 2020, And Astronomers Have Just Found a New Class of Massive Space Explosions - ScienceAlert - June 9th, 2020
- The Brilliant Astronomer Who Devised New Tactics to Fight Anti-Gay Bias - The New York Times - June 9th, 2020
- Gamma Rays Detected Coming From the Crab Nebula - Universe Today - June 9th, 2020
- The Sky This Week from April 24 to May 1 - Astronomy Magazine - April 26th, 2020
- A century ago, astronomys Great Debate foreshadowed todays view of the universe - Science News - April 26th, 2020
- Hubble watches a suspected exoplanet disappear before its very eyes - Astronomy Magazine - April 26th, 2020
- Four amazing astronomical discoveries from ancient Greece - The Conversation UK - April 26th, 2020
- Astronomers Just Identified 19 More Asteroids They Think Are Interstellar - ScienceAlert - April 26th, 2020
- Astronomers May Have Captured the First Ever Image of Nearby Exoplanet Proxima C - Scientific American - April 26th, 2020
- Travis Scott will launch 'Astronomical' into the Fortnite frontier this week - Space.com - April 26th, 2020
- Astronomy tips: How to photograph the moon, stars, and sky - Los Angeles Times - April 26th, 2020
- Astronomers Took New Pics of 1998 OR2, The Asteroid About to Whoosh Past Earth - ScienceAlert - April 26th, 2020
- Astronomers Detected a Black Hole Merger With Very Different Mass Objects - Universe Today - April 26th, 2020
- Everything You Need to Know to Take up Stargazing - Thrillist - April 26th, 2020
- Astronomers Discover The Science Behind Star Bursts That Light Up The Sky - Scoop.co.nz - April 26th, 2020
- Blowtorch of the Gods Captured by Black Hole Image Makers - The New York Times - April 9th, 2020
- E. Margaret Burbidge, Astronomer Who Blazed Trails on Earth, Dies at 100 - The New York Times - April 9th, 2020
- Astronomers are hoping to see the very first stars and galaxies in the Universe - Universe Today - April 9th, 2020
- Marvel at the universe with the free Northeast Astronomy Forum Virtual Experience today! - Space.com - April 9th, 2020
- Here's the best way to enjoy the 'Super Pink Moon,' according to a NASA astronomer - Space.com - April 9th, 2020
- Astronomers spot never-before-seen gravitational wave source from binary white dwarf stars - Space.com - April 9th, 2020
- Interview: Jim Lovell relives the successful failure of Apollo 13 - Astronomy Magazine - April 9th, 2020
- See the waning crescent Moon meet the dawn planets, 1516 April 2020 - Astronomy Now Online - April 9th, 2020
- View On Astronomy: Want to see Betelgeuse supernova? You'll have to wait a bit longer - The Independent - April 9th, 2020
- Comets Are Breaking Apart in Our Cosmic Backyard, and Astronomers Are Stoked - Popular Mechanics - April 9th, 2020
- South African Radio Astronomy Observatory Mandated To Manage The Production Of Respiratory Ventilators - Space in Africa - April 9th, 2020
- COVID-19 Forces Earth's Largest Telescopes to Close. But a Few Isolated Astronomers Are Still Watching Over the Cosmos - Discover Magazine - April 9th, 2020
- Supermoon and a meteor shower: 2 astronomical events in April to watch from your backyard - LancasterOnline - April 9th, 2020
- Photos: Venus and the Pleiades prepare to meet | Astronomy Essentials - EarthSky - April 9th, 2020
- The Thirty Meter Telescope: How a volcano in Hawaii became a battleground for astronomy - Space.com - March 26th, 2020
- The Sky This Week from March 20 to 27 - Astronomy Magazine - March 26th, 2020