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 definitely 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 unlimited 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 traditional knowledge, it’s not an eons-old leftover from the solar system’s formation. That primordial dust is lengthy gone. Right now’s interplanetary dust is – in an astronomical sense – very younger, solely thousands and thousands of years previous. A lot of the particles originate from fairly current incidents, like asteroid collisions.
Uncovered to varied 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 a tendency to collide with bigger our bodies or slowly spiral into the inside solar system the place they fall into the sun.
Nonetheless, asteroid collisions and the erosion of comets always replenish the availability of interplanetary dust. So, the interplanetary dust cloud of the solar system is just not 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. Typically referred to as the false daybreak, the zodiacal gentle is sort of shiny and could be seen from any observing web site 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 shiny 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. In contrast to the zodiacal gentle, nonetheless, these sections are very onerous to detect since they’ve an especially 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 realm of the gegenschein.
Brighter when reverse the sun
However why does the gegenschein glow brighter than the remainder of the zodiacal band? Trying 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 will 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 cause for this brightening throughout opposition is shadow hiding, that means that every one particles are absolutely illuminated. You possibly can see this opposition impact right here on Earth throughout daylight should you’re outdoor and take 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 picture 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 is just not 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 on account of 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 images classes at dark-sky sites, the gegenschein was a well-known companion. Out beneath the celebrities, ultimately one among us would point out:
Hmm, the gegenschein is sort of apparent tonight…
… adopted by the standard reply:
Yep, shiny 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 previous dark-sky companion, the gegenschein. However for some cause it by no means got here to it. Different tasks had been in the way in which, on different events the place within the sky was not proper, or the environment’s transparency was simply not ok.
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 had 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 had 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 stage, we had the right circumstances 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. Under 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 man-made marking of the ecliptic. Below these circumstances, taking pictures the gegenschein was a straightforward job.
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 gadget, the Mini Monitor LX2. This modern gadget is a frequent companion on our imaging excursions. For these fascinated about extra particulars, we offer a thorough review of the Mini Monitor 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 had been later calibrated, registered and stacked with DeepSkyStacker. Processing of the ultimate picture was executed in Photoshop, the place we utilized substantial distinction enhancement.
How one can see the gegenschein
When you’ve by no means encountered the gegenschein your self, don’t anticipate 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 most effective circumstances, 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 shiny because the dimmest components of the winter Milky Way. The stretch of the Milky Way that approximates the counterglow’s brightness greatest is the part between the star Mirfak and the asterism referred to as the Youngsters subsequent to Capella. That’s very dim. Any shiny object in its neighborhood (e.g. Jupiter or Mars at opposition) makes it onerous to look at.
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.
Comply with these 5 tricks to see the gegenschein for your self.
Tip 1: Verify the place
Earlier than you start looking for the gegenschein within the night time sky, you must 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 depends upon 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 brilliant 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 will outshine it.
Some planetarium applications or stargazing apps may also be helpful instruments to find out the place of the gegenschein. If the software program gives 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 most effective time of the night time to provide an statement a attempt. There must also be no moon above the horizon. However that’s not all. Just like the sun itself, the antisolar level too has totally different fruits heights through the course of the yr.
For observers within the Northern Hemisphere, the antisolar level has its highest fruits 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 most effective months to see the gegenschein from northern latitudes are November and January.
For Southern Hemisphere observers, the most important fruits peak of the antisolar level occurs at summer time 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 Could or August.
Tip 3: Strive from the most effective observing location you have got entry to
You possibly can’t see the gegenschein from light-polluted websites. Even reasonable gentle air pollution diminishes the counterglow’s distinction an excessive amount of. Based mostly 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 conversant in the gegenschein and know precisely the place to look.
For first-time observers, we advocate a web site with a sky brightness of 21.2 magazine/arcsecond2 within the zenith or higher. These circumstances solely exist removed from cities or different inhabited areas. Observing websites within the mountains particularly qualify due to the diminished air mass at larger altitudes above sea stage. (Click here to find a dark-sky site near you.)
Tip 4: Await favorable climate circumstances
Even should you observe from a dark-sky location, the typical clear sky may not be ok for seeing the gegenschein. What you actually need is an evening sky with exceptionally excessive transparency. This can solely be the case if the air mass above your observing web site is as dry as doable.
To get an thought of the present scenario at your location, you need to 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 could be that the air is clear, i.e. freed from dust. At mid-latitudes, the required circumstances of dry and clear air typically happen after the passage of a chilly entrance or a trough.
Tip 5: Develop your observing expertise
Given you have got good eyesight at midnight and that you have already got some observing expertise, there are a number of observing methods it’s essential to grasp.
First, it’s essential to learn to turn out to be really darkish tailored. This may put your persistence to a check, since your eyes want at the very least 3/4 of an hour to completely modify to the darkish. So, no smartphones or different handheld gadgets throughout this time. It is best to solely use very dim crimson lights should you don’t need to destroy 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 a bit of off to the facet whereas persevering with to focus on the article. This manner you might be 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, searching down the gegenschein will enhance your observing expertise rather a lot. Apart from, it makes a extremely attention-grabbing challenge and ultimately you, too, will have the ability to put that prized stargazer’s trophy in your shelf.
What’s Mission 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 pictures and tales, we elevate consciousness for the necessity to preserve gentle air pollution at bay. Our crew is predicated in Vienna, Austria, and consists of skilled, world-wide energetic astrophotographers who work on a volunteer foundation. Most of our energetic members are advertising or communication professionals. If you wish to be taught 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.