The sun’s counterglow – or gegenschein – is a stargazers’ holy grail. Each beginner astronomer has heard of it. However only some of them have ever seen it. And fewer nonetheless have captured a picture of this dim and ghostlike apparition. As a fellow observer put it:
The gegenschein is actually not a GoTo object.
Matter of reality, it isn’t an object in any respect. However let’s begin from the start.
What’s the gegenschein?
The space between the planets isn’t empty. The airplane of the solar system is crammed with an infinite disk of small dust particles with sizes starting from lower than 1/1000 of a millimeter as much as 1 millimeter. This interplanetary dust cloud is a extremely dynamic construction. In distinction to standard knowledge, it isn’t an eons-old leftover from the solar system’s formation. That primordial dust is lengthy gone. Right this moment’s interplanetary dust is – in an astronomical sense – very younger, solely hundreds of thousands of years outdated. A lot of the particles originate from fairly current incidents, like asteroid collisions.
Uncovered to numerous forces, the particles do not stay in stationary orbits however inhabit the disk in ever-changing movement. The smallest particles (lower than 1/1000 of a millimeter) are swiftly blown out of the solar system by the solar wind. The bigger particles don’t survive lengthy both. They have an inclination to collide with bigger our bodies or slowly spiral into the interior solar system the place they fall into the sun.
Nonetheless, asteroid collisions and the erosion of comets always replenish the provision of interplanetary dust. So, the interplanetary dust cloud of the solar system isn’t a static formation however a dynamic construction consisting of fairly younger elements.
Zodiacal gentle and gegenschein
For an observer on Earth, this dynamic dust cloud is principally seen within the type of the zodiacal light. After nightfall and earlier than daybreak it extends as a cone of sunshine from the sun alongside the ecliptic path. Usually referred to as the false daybreak, the zodiacal gentle is kind of brilliant and could be seen from any observing website that doesn’t undergo from extreme gentle air pollution, particularly when the ecliptic is excessive within the sky. For Northern Hemisphere observers, that is the case within the night sky throughout spring and within the morning sky throughout autumn.
Apparently, recent research suggests that the zodiacal gentle could also be from Martian dust storms.
The morning and night zodiacal gentle are the 2 areas of the interplanetary dust disk the place daylight will get ahead scattered to earth, ensuing within the brilliant silvery gentle cones. The interplanetary dust disk can also be seen alongside the remainder of the ecliptic path, the place it is called the zodiacal band. Not like the zodiacal gentle, nevertheless, these sections are very exhausting to detect since they’ve a particularly low floor brightness. However on the level immediately reverse the sun the geometry once more works in our favor and enhances the visibility of the interplanetary dust. That is the world of the gegenschein.
Brighter when reverse the sun
However why does the gegenschein glow brighter than the remainder of the zodiacal band? Wanting on the interplanetary dust disk from Earth, the part on the antisolar point is illuminated from immediately behind our heads. This ends in a rise in brightness referred to as the opposition effect. The opposition impact is a frequent phenomenon for solar system our bodies. For instance, the moon exhibits a major peak in brightness round full moon, i.e. at its opposition. One other instance is Saturn’s rings within the days around opposition, once they brighten drastically.
You’ll have additionally seen this impact when on an airplane. Read more: How can you see an airplane glory?
The gegenschein impact on Earth
The principle purpose for this brightening throughout opposition is shadow hiding, which means that each one particles are totally illuminated. You possibly can see this opposition impact right here on Earth throughout daylight in the event you’re open air and have a look at the bottom in entrance of you. If the bottom is sufficiently coarse, you’ll see a brightening across the shadow of your head. To doc this phenomenon, we made the photograph beneath, which exhibits the impact on volcanic gravel.
Apparently, the brightening across the digicam’s shadow had an obvious diameter of 10 levels, about the identical measurement because the diameter of the gegenschein within the night time sky. Subsequent time you stroll on a tough floor in daylight, search for this terrestrial model of the counterglow.
To sum up, the gegenschein isn’t an object. It’s a play of sunshine on the solar system’s interplanetary dust disk. The part of the dust on the antisolar level is squarely illuminated, leading to a brightness enhancement resulting from shadow hiding. This so-called opposition impact could be encountered at many events, however with the gegenschein it makes its most ghostlike and outlandish look. That is most likely the explanation why it’s such a prized trophy amongst beginner astronomers.
Gegenschein beneath darkish skies
From our observing and pictures classes at dark-sky sites, the gegenschein was a well-recognized companion. Out beneath the celebs, in the end one among us would point out:
Hmm, the gegenschein is kind of apparent tonight…
… adopted by the everyday reply:
Yep, brilliant and glossy!
This quick dialogue would point out that our eyes had reached full dark adaptation and our night time imaginative and prescient was at most capability. We frequently talked about making a picture of our good outdated dark-sky companion, the gegenschein. However for some purpose it by no means got here to it. Different initiatives have been in the way in which, on different events the place within the sky was not proper, or the ambiance’s transparency was simply not adequate.
How we captured the gegenschein
However on the night time from October 30 to 31, 2019, we lastly did give it a go. On October 30 we have been already a number of days on an imaging tour on La Palma island. Throughout daytime the annoying Calima weather pattern had lastly stopped and skies have been once more as deep blue and clear as they are often on this stunning island. Accommodated in a solitary finca far-off from inhabited areas and about 800 meters (2,600 ft) above sea degree, we had the right situations for photographing the gegenschein.
Round midnight, when the counterglow culminates, our devices registered a sky brightness of 21.4 magnitudes per sq. arcsecond within the zenith. That is so darkish that Sirius noticeably brightened the panorama when it rose behind a mountain ridge later that night time.
The gegenschein itself was fairly apparent to the unaided eye. Beneath the constellation Aries, immediately on the ecliptic path, a definite glow about 10 levels throughout marked the antisolar level. The very faint zodiacal band crossing the entire sky was seen too, virtually wanting like a synthetic marking of the ecliptic. Below these situations, taking pictures the gegenschein was a straightforward activity.
Particulars, particulars
For these within the technical particulars: We made 23 unfiltered exposures of 4 minutes with a 16mm lens @f/5.6 on a Baader-modified 1100D Canon DSLR. For sky monitoring we used a purely mechanical machine, the Mini Observe LX2. This revolutionary machine is a frequent companion on our imaging excursions. For these enthusiastic about extra particulars, we offer a thorough review of the Mini Observe LX2 for obtain on our website.
The total weight of our imaging gear was lower than 3 kilograms (6.6 kilos), together with digicam and tripod. The 23 particular person exposures of the gegenschein have been later calibrated, registered and stacked with DeepSkyStacker. Processing of the ultimate picture was performed in Photoshop, the place we utilized substantial distinction enhancement.
How one can see the gegenschein
In case you’ve by no means encountered the gegenschein your self, don’t count on it to be as distinguished as within the {photograph} on the high of this put up. We utilized heavy distinction enhancement to the picture to make the counterglow, its form and its measurement higher seen. To our human eyes, even beneath the very best situations, the gegenschein is an elusive and dim glow.
On a number of events we in contrast the visible brightness of the counterglow and sections of the Milky Way. We at all times discovered it to be about as brilliant because the dimmest components of the winter Milky Way. The stretch of the Milky Way that approximates the counterglow’s brightness finest is the part between the star Mirfak and the asterism referred to as the Youngsters subsequent to Capella. That’s very dim. Any brilliant object in its neighborhood (e.g. Jupiter or Mars at opposition) makes it exhausting to watch.
Except for being barely brighter within the center and fading out uniformly in all instructions, the gegenschein exhibits no construction in any respect. It has a roundish kind, round or elliptical, measuring roughly 10 levels throughout. That’s in regards to the measurement of a fist stretched out at arm’s size.
Observe these 5 tricks to see the gegenschein for your self.
Tip 1: Verify the place
Earlier than you start trying to find the gegenschein within the night time sky, it’s best to examine its present place. The map beneath helps you understand the place to look. It additionally exhibits you whether or not the counterglow is detectable in any respect, as a result of its visibility extremely will depend on its place in opposition to the starry background.
In June, July and December it’s utterly invisible, as a result of it’s in entrance of the Milky Way. Different obstacles could be the intense planets Mars and Jupiter. When one among them is close to opposition, it’s in the identical area of the sky because the counterglow and should outshine it.
Some planetarium packages or stargazing apps may additionally be helpful instruments to find out the place of the gegenschein. If the software program offers an choice to show the middle of Earth’s shadow, this can present you the present antisolar level.
Tip 2: Search for it at most peak above the horizon
The gegenschein is at all times highest above the horizon round native midnight, in order that’s the very best time of the night time to present an remark a strive. There also needs to be no moon above the horizon. However that’s not all. Just like the sun itself, the antisolar level too has totally different end result heights in the course of the course of the yr.
For observers within the Northern Hemisphere, the antisolar level has its highest end result on the winter solstice round December 21. Sadly, throughout December the gegenschein is in entrance of the winter Milky Way and subsequently invisible. So, the very best months to see the gegenschein from northern latitudes are November and January.
For Southern Hemisphere observers, the most important end result peak of the antisolar level occurs at summer season solstice round June 21. For southerners too, the Milky Way is in the way in which. Once more, one ought to look earlier than or after the solstice, on this case throughout Might or August.
Tip 3: Strive from the very best observing location you’ve got entry to
You possibly can’t see the gegenschein from light-polluted websites. Even average gentle air pollution diminishes the counterglow’s distinction an excessive amount of. Primarily based on our observations, absolutely the minimal to detect the gegenschein is a sky brightness of 21.0 magazine/arcsecond2 within the zenith. However this is applicable solely if you’re already fairly accustomed to the gegenschein and know precisely the place to look.
For first-time observers, we advocate a website with a sky brightness of 21.2 magazine/arcsecond2 within the zenith or higher. These situations solely exist removed from cities or different inhabited areas. Observing websites within the mountains particularly qualify due to the decreased air mass at increased altitudes above sea degree. (Click here to find a dark-sky site near you.)
Tip 4: Anticipate favorable climate situations
Even in the event you observe from a dark-sky location, the typical clear sky may not be adequate for seeing the gegenschein. What you really need is an evening sky with exceptionally excessive transparency. This can solely be the case if the air mass above your observing website is as dry as doable.
To get an concept of the present scenario at your location, you should use a web site that gives climate forecasts for astronomical observations (e.g. 7timer.info). Websites like this show knowledge about humidity in any respect tropospheric layers. One other situation for prime transparency can be that the air is clear, i.e. freed from dust. At mid-latitudes, the required situations of dry and clear air typically happen after the passage of a chilly entrance or a trough.
Tip 5: Increase your observing expertise
Given you’ve got good eyesight in the dead of night and that you have already got some observing expertise, there are a number of observing strategies you should grasp.
First, you should learn to change into actually darkish tailored. This may put your persistence to a take a look at, since your eyes want not less than 3/4 of an hour to completely modify to the darkish. So, no smartphones or different handheld units throughout this time. You must solely use very dim crimson lights in the event you don’t wish to wreck your night time imaginative and prescient.
A second approach you may want for seeing the gegenschein is averted imaginative and prescient. With averted imaginative and prescient, you don’t look immediately at an object however just a little off to the facet whereas persevering with to focus on the thing. This manner you’re utilizing peripheral imaginative and prescient, which is extra delicate to low gentle ranges than the middle of the attention. Some observers report that this method makes a giant distinction for them, and it’d enable you too. In any case, looking down the gegenschein will enhance your observing expertise rather a lot. In addition to, it makes a extremely fascinating challenge and in the end you, too, will be capable to put that prized stargazer’s trophy in your shelf.
What’s Challenge Nightflight?
About Project Nightflight: “Our mission is the starry sky. We internationally promote the conservation of the starry sky as environmental useful resource. To assist this objective, we work collectively carefully with our sponsors and worldwide information media. With our photos and tales, we elevate consciousness for the necessity to maintain gentle air pollution at bay. Our group relies in Vienna, Austria, and consists of skilled, world-wide lively astrophotographers who work on a volunteer foundation. Most of our lively members are advertising and marketing or communication professionals. If you wish to study extra about our group, please obtain the Project Nightflight profile.”
Backside line: The gegenschein is a play of sunshine on interplanetary dust of the solar system. It’s a diffuse path of sunshine that lies on the antisolar level, the purpose precisely reverse the sun.
