Category Archives: astronomical phenomena

Noctilucent clouds in February?

On February 20 and 24, 2013, unusual clouds which looked like NLC (noctilucent clouds) were observed by the pilots Terry J. Parker above Birmingham, UK, Nikolay N. Nikolaev and Egor C. above Moscow, Russia (picture at top).

In the AKM forum (AKM = Arbeitskreis Meteore/ Meteor Workshop) there were discussions on what could have been the reason for this unusual phenomenon.

  • Polar Stratospheric Clouds can be excluded as a reason because the stratosphere was too warm at the time the observations were made (12).
  • MAARSY (Middle Atmosphere Alomar Radar System) in Andenes, Norway, recorded strong echoes in the mesosphere. Weak echoes have also been recorded at Kühlungsborn, Germany. Mesospheric winter echoes are common, but up to now there are no special observations recorded which were connected to these echoes.
  • There was a full moon on February 25, so high cirrus clouds illuminated by the moon cannot be excluded as a reason.
  • The most probable explanation for the phenomenon is, however, that the clouds had been caused by the meteor which hit near Cheljabinsk, Russia, on February 15. A recent study by Kathryn Hansen shows that this cloud of smoke travelled around earth in an altitude of about 40 kilometres. This fits with altitude measurements made in Wales, which showed that the unusual clouds had formed at altitudes between 35 and 38 kilometres.

Another article to this Topic:  “Noctilucent clouds in october ?”

The black drop effect is not an atmospheric phenomenon

The so-called black drop effect is an optical phenomenon which can be seen during transits of Mercury or Venus in front of the sun. It can only be observed through a telescope protected against the bright sunlight. When the planet begins to cover the sun, it seems as if the silhouette of the planet would form a kind of black drop when it detaches from the rim of the solar disk. The same effect appears again when the silhouette touches the rim of the solar disk at the end of the transit. It looks as if the planet merges with the rim of the solar disk like two converging drops of water.

Originally astronomers thought that this phenomenon was caused by different refraction of light in the atmospheres of the planets. But today we know that the phenomenon is caused by the limited resolving capacity of the telescopes used. In this context experts often refer to an experiment which everybody can realize using his own fingers:

Just form a ring with your thumb and your trigger finger, but exactly so that the fingers just do not touch each other. Hold this narrow gap in font of your eyes, so near that  they cannot focus it. A “shadow bridge” appears between the fingers, especially when the fingers are held in a different distance from the eyes and you start closing the gap by changing the perspective. The shadow bridge then moves from the finger which is further away from your eyes to the closer one.

Shadow bridge between thumb and trigger finger. The camera had been focused behind the fingers

The gap between the fingers has been exactly focused a no shadow bridge appears

Important for the successful execution of this experiment is that your eyes are defocused. If you move the fingers away from your eyes so that they can focus them, the shadow bridge completely disappears.

I slightly modified and analyzed this simple experiment. Instead of two fingers, I only used one, but in front of a pattern of blue and white stripes.

Shadow bridge experiment No.2: Heavily defocused photograph of my trigger finger in front of a background of blue and white stripes

With this method I observed two sources of fuzziness , which are the silhouette of the dark brown finger and that of the stripes. In front of the dark stripes, the area of fuzziness of the finger appears more tangent than in font of the white ones. This gives the impression of the finger being as double as wide in front of the blue stripes compared to the white ones. In reality, however, the fuzziness of the finger is always the same as I tried to keep it parallel to the background and perpendicular to the line of sight. The most interesting area is that where the fuzziness of the finger meets the fuzziness of a blue line. There it also causes a deeper tinting of the blue area. As a consequence, a kind of dark “mound” forms in the zone of fuzziness of the blue area which points in direction to the finger tip. Moving the finger so, that its silhouette touches the outer rim of the fuzziness between the blue and the white line makes a shadow bridge appear.

Completed “shadow bridge”

Using this knowledge, you can easily simulate a transiting planet yourself. The experiment is very simple. Just draw a white circle with a black background on your computer and print it. Then die-cut a circle out of a sheet of black paper using a hole puncher. You only need the chad to represent the planet. Put this on a clear CD-cover and put this onto your printed solar disk, so that the planet lies as near to the rim of the solar disk as possible. Here are an animation and two photos illustrating the black drop effect, an exactly focused one with no black drop effect, and another, defocused one, in which the black drop effect appears.

Simulated black drop effect. The picture on the left is exactly focused and shows no “shadow bridge”. The photograph on the right is defocused and shows a “shadow bridge”

Author: Reinhard Nitze, Barsinghausen, Germany

Etruscan Vase during Venus Transit

At sunrise during the transit of Venus on June 6, 2012, there were not only distortions of Sun and Venus visible as well as the Green Flash, but there were also several observations of the so called Etruscan Vase.

As weather forecasts for Germany´s sunniest island, Fehmarn in the Baltic Sea, were most favorable for that day, Jens Hackmann flew there from the bad weather of his home town Bad Mergentheim. Just after sunrise at 4.41 hours, he observed the mirage effects mentioned above not only on Sun and Venus, but also on a passing ship. And only a few moments later the Etruscan Vase phenomenon appeared, an upside-down mirage of the sun which appears in most cases above a water surface (more pics and film).

Thomas Stemmler photographed the transit of Venus at beach on the Baltic Sea near Dahme and could shot mirage effects and the phenomena of Etruscian vase also.

This strange effect is caused by the refraction of sunlight together with a lower mirage and appears when a layer of cold air is positioned over a warm water surface. The lowermost air layer, which is heated up by the warm water, has a lower refraction index than the air at the level of the eyes of the observer. Sunrays hitting this layer in a very sharp angle can be reflected totally. So the observer does not only see light coming directly from the sun, but also light that had been reflected by the warmer and less dense layer of air directly above the water. The rays coming directly from the sun let the sun appear totally normal. But as our brains are not programmed for totally reflected sunrays, they extrapolate them lineally. This makes us see an upside-down reflection of the sun beneath the real sun which changes due to the angle of incidence of the light and thus with the sun elevation.

This phenomenon reminded the science-fiction-author Jules Verne of a paunchy Etruscan Vase standing on a pedestal, so he coined this term for the phenomenon.

Air mirage effects during transit of Venus

On June 06, 2012, the rare constellation of the planet Venus crossing through the solar disk could be observed from the day side of the earth.

When in Germany the sun rose, the Venus transit was already in its final stage. Where the sun passed through differently dense air layers during its rise, the mirage phenomenon was visible, i.e. the sun as well as Venus appeared distorted or as a combination of multiple images. These effects are due to the different bending of the light waves at air layers of varying density. Moreover, an incident ray of light will be reflected at the interface between cool and warm air. When there are more than one of these interfaces, multiple reflection might occur.

Rico Hickmann could even observe the Green Flash during the Venus transit from Dresden: “I was incredible lucky with respect to the weather. Yesterday evening, it was still very cloudy, and after the end of the transit there were again clouds filling the sky. Before sunrise, a light pillar could be seen, that served as a pointer towards the sun. The sunrise over Dresden was spectacular, Green Flashes and disconnected segments… I’m still speechless.” Here are some more incredible pictures from this series: 12345

Another spectacular image of a triple Venus was obtained by Frank Killich, who observed the Venus transit from the Wolfswarte in the Harz mountains (916 m / 0 °C). The image is a single frame from a HD video file.

Some more and equivalently wonderful observations were reported.  Alexander Haussmann made a Video showing a triple Venus, green segments and distorted Venus passing through different air layers that were responsible for the green segments some seconds before. Further examples of impressive green flashes are the pictures of Andreas Möller, taken in Zinnowitz (photo and and animation as gif or MP4 [better quality]) and Hermann Koberger from Fornach, Austria (123).

Sirius scintillation

Caused by the numerous cities and industrial areas south of my observation site (North-Rhine Westfalia, Ruhr Area, Germany), there are rather strong air turbulences (bad seeing) near the horizon. But what is bad for astronomical photographs, however, can be very nice to demonstrate atmospheric aberration and dispersion in the star trails on photographs without tracking. The lower a star is in the sky, the more pronounced is this effect, especially at very bright stars.

In this case, Sirius had an elevation of 11° on October 23, 2011, at 3.10 hours. The sky was clear, wind was at 1-2 Bft, temperature 3°C and humidity at about 80%.

I took this photograph using a Canon EOS 350D, which was focally adapted to a Maksutov (6”, 1800mm, f 12,0). After having been adjusted and properly focused, the telescope was driven at maximum speed over the right ascension axis. This makes the star transit rapidly through the field of view causing a star track on the camera chip which records the chronological sequence of the flickering of the star.

Author: Ronald Blendeck, Germany

Ozone makes twilight wedge blue

Normally, the twilight wedge appears in a rather grey colour. But sometimes, in most cases above an inversion layer, the twilight wedge appears tuquoise-coloured as during this morning twilight on March 2, 2011. The night before had been so clear that the zodiacal light was visible for the naked eye. Towards the sun, above the first light of dawn, the crescent of the moon and Venus made a nice contrast to the blue sky. On the opposite side, there was a definite twilight wedge showing a rarely clear blue colour. This colour is caused by light absorption in the ozone layer.
Above the twilight wedge there was also the venus belt visible. (Photos 1234)

Author: Claudia Hinz, Brannenburg, Germany

Total Lunar Eclipse on 16th June 2011

On June 16th, 2011, a total lunar eclipse, centered on the Indian Ocean, was visible from Europe, Africa, and Asia. The photo shown above was taken a short time after moonrise by Wolfgang Hinz in the Bavarian Alps, Germany.

Further pictures sent Pitan Singhasaneh, taken in Bangkok, Thailand, where the total phase happened just before the Moon set.

The eclipse was quite dark in all regions of visibility. Therefore, the Moon was only seen as a feebly lit reddish-brown disk during the never quite dark late summer evening hours in central Europe. The remaining illumination on the Moon is due to light refracted into the Earth’s umbra in different layers of its atmosphere, and therefore bears the imprint of the red-orange color of the skies at the Earth’s dawn or dusk terminators. Because the Moon travelled almost right through the umbra’s center, this eclipse had a very long duration of totality (101 minutes) and consequently became quite dark in the innermost parts of the umbra which only get light from the lowermost and thickest layers of the Earth’s atmosphere. Additionally, the atmosphere was contaminated by dust from three volcanic events in Chile, Iceland, and Ethiopia which might have further diminished the illumination of the eclipsed Moon.

Elmar Schmidt did photometry on the Moon at Farm Hakos in Namibia at 1834 m altitude. The darkness of the eclipse was quite pronounced even under the excellent skies of Namibia, where the Moon climbed to an elevation of more than 50 degrees during mid-totality. Then, a pair of 10×40 binoculars did not show the contours of the lunar maria at the brownish grey center of the Moon’s disk. On the other hand, the eclipsed Moon always retained a generally deep orange color. Its minimum brightness of -0.35 visual astronomical magnitudes complies with an extrapolation from less deeper eclipses, thus not really pointing to a significant influence of volcanic dust. An exception could be taken with respect to the asymmetry of the eclipse which was much darker at the exit as compared to the ingress – although more colorful because of showing the blueish-green tinge at the umbra’s rim. This might hint to darker or altered atmospheric conditions near the Earth’s eastern terminator over the Indian Ocean and Western Australia.

Authors: Claudia Hinz, Elmar Schmidt

Perigee Moon

This photo was taken by Claudia Hinz at the evening of Jan. 11th, 19.35 CET from Mt. Wendelstein (1838m), Southern Germany. The full Moon in this night was extra bright. Dr. Elmar Schmidt of the SRH University of Applied Sciences in Heidelberg, Germany, used an absolutely-calibrated photometer to precisely measure the moonlight and found it more than 50% brighter than that of a typical full Moon.

1. The Moon was at perigee, the side of the Moon’s elliptical orbit closest to Earth.

2. The Earth-Moon system was near perihelion, the side of Earth’s elliptical orbit closest to the sun. Extra sunlight increased the reflected luminosity of the Moon.

3. The Sun-Earth-Moon trio were almost perfectly aligned. This triggered a strong opposition effect an intense brightening of the lunar surface caused by the temporary elimination of normal shadows.

4. The weather conditions were optimal for photometry due to the clean and dry arctic air (its relative humidity being less than 10% at the moment of the photo). This resulted in only clear air scattering of moonlight with no extraneous glare as evident in the completely blue night sky. The brightness of the mountain landscape was additionally increased because of the reflection from the snow.

Elmar Schmidt details the relative contributions of each factor in his full report.

Authors: Elmar Schmidt & Claudia Hinz