Published in Sky&Telescope 140(4), 11 (October issue) in 2020
A.G. de Wijn and G.P. Können
We recorded the passing of Venus over the solar corona during its very narrow inferior conjunction of 3 June 2020. The instrument used was the K-Cor coronagraph situated at Mauna Loa Observatory (Hawaii). silhouette of Venus. The movement of the silhouette of Venus across the corona is shown in two time-lapse movies and four stills. See 
Published in Appl. Opt. 59 F1-F10 in 2020
G. P. Können, G. Schneider, E. H. Zucker, P. Lahtinen
A photographic observation sequence was obtained of a subsun before, during and after the total phase of the 2016 solar eclipse. The time-resolved images were obtained from a high-altitude jet aircraft. The image sequence was searched for the possible presence of a solar corona-generated subsun during totality. Although the subsun-creating conditions apparently persisted during totality, the drop in signal intensity compared to the local background prevented its detection. Separately, we document a visual observation from the 1977 total solar eclipse of a rainbow that faded, in the last a few seconds before totality, from as normally multicolored to monochromatic red from water drops then predominantly illuminated by light from the solar chromosphere. A similar transition in the final seconds before, and after, totality are expected occur for parhelia. The posited short-living monochromatic red parhelion resulting from the momentary illumination of ice crystals by the solar chromosphere is still waiting to be observed.
Published in Bull. Amer. Meteor. Soc. 98 485-494 in 2017
G. P. Kónnen
Rainbows, halos, coronas, and glories are aesthetic features admired over millennia by man. They are also carriers of useful information about the physical properties of drops and solid particles floating in the atmospheres of Earth and other planets. Although atmospheric optics is a centuries-old discipline, significant progress in the understanding of these phenomena has been achieved during the past 40 years, as well as of their relation to the physical properties of the individual scatterers. At the same time, with the development of planetary exploration, the number of observations of rainbows, halos, and glories in the atmospheres of planets other than Earth is steadily growing. In this article we describe these phenomena and their information content and illustrate them with pictures mostly showing their typical appearance rather than presenting the “best pictures ever.”
Published in Appl. Opt. 54, B1-B7 in 2015
G.P. Können, J. Tinbergen, and P. Stammes
Sirius was spotted with the naked eye at broad daylight by looking along the finder of a 1 m telescope on La Palma Observatory at a 2370 m height. Sun elevation was 73°; Sirius was nearly straight under the Sun at 37° elevation. The sky radiance, although not recorded directly, could be determined from the simultaneously obtained high-precision wavelength-dependent sky polarization data near Sirius. This was done by fitting the polarization data with the doubling-adding KNMI (DAK) radiative transfer model, which provided the values of the surface albedo and of the aerosol optical thickness required for determining the absolute sky radiance. Our analysis implies that Sirius, when positioned overhead, can be a daytime naked eye object from sea level even if its culmination occurs at solar noon. It also suggests that the second-brightest star (Canopus), if positioned overhead, could be perceptible even at solar noon.
Published in Private Document 2011/01 in 2011
G. P. Können
There is some confusion about the nationality of the Belgium-born Dutch astronomy professor M. Minnaert (author of ‘Light and Colour in the Open Air’). Historically, the best way is to call him either a ‘Dutch astronomer’ or a ‘Belgium-born Dutch astronomer’.
Published in JPL in 2009
At the IAU meeting of December 2009 Minor Planet 12157 (an asteroid) received the name Können, after me. Its diameter is about 5 km. Asteroid Können is at 2.4 AE from the sun and circles the sun in 3.75 year. The naming is formalized on 2 Dec 2009.
Published in Appl. Opt. 47, H14-H24, doi:10.1364/AO.47.000H14 in 2008
Gunther P. Können and Claudia Hinz
The limiting magnitude during totality is +3.5. Diffraction coronas and even halos around the totally eclipsed sun may nevertheless occur. Rainbows during totality seem impossible.
Published in Weather 62, 166-167, doi: 10.1002/wea.100 in 2007
G. P. Können
A response is given to a Letter by G. Watts questioning the identification of subsuns in general and of the bright steak on Mars in terms of halo scattering.
Published in Weather 61, 171-172, doi:10.1256/wea.46.06 in 2006
G. P. Können
A bright subsun is spotted on a satellite picture of Mars. To our knowledge this is the first time that a halo is identified on a picture of a planet other than the Earth.
Published in K. Fletcher (Ed) Titan, from discovery to encounter. ESA SP-1278, Noordwijk Netherlands, 323-330 in 2004
G. P. Können
The prospects of the Huygens probe to detect during its descent halos from methane or ethane crystals on Titan are discussed. Diagrams of potential halo displays on Titan are shown.
Published in Appl. Opt. 42, 318-331, doi:10.1364/AO.42.000318 in 2003
Gunther P. Können
It is found that halo displays are always left-right (L-R) symmetric if the crystals are formed from the surrounding vapor. This leaves room for two types of halo display only: a full symmetric one (mmm -symmetric), and a partial symmetric one (mm2 -symmetric) in which halo constituents lack their counterparts on the other side of the parhelic circle. Partial symmetric displays can occur only for point halos and only if the halo-making crystals lack a center of inversion, any rotatory-inversion axis that is parallel to the crystal spin axis P, a mirror plane perpendicular to the P axis, and a twofold rotation axis perpendicular to the P axis. A simple conceptual method is presented to reconstruct possible shapes of the halo-generating crystals from the halos in the display. Halos that may occur on the Saturnian satellite Titan are briefly discussed.
Published in Appl. Opt. 42, 367-378, doi:10.1364/AO.42.000367 in 2003
Siebren Y. van der Werf, Günther P. Können, Waldemar H. Lehn
Systematics of the Novaya Zemlya (NZ) effect are discussed in the context of sunsets. We distinguish full mirages, exhibiting oscillatory light paths and their onsets, the subcritical mirages. Ray-tracing examples and sequences of solar images are shown. We discuss two historical observations by Fridtjof Nansen and by Vivian Fuchs, and we report a recent South Pole observation of the NZ effect for the Moon.
Published in Appl. Opt. 42, 379-389, doi:10.1364/AO.42.000379 in 2003
Siebren Y. van der Werf, Günther P. Können, Waldemar H. Lehn, Frits Steenhuisen, and Wayne P.S. Davidson
Ray-tracing analyses show that the NZ effect distorted the relative positions of Jupiter and the Moon in such a way that the looked-for fingerprint of the 1597 conjunction occurred almost 2 h after the true conjunction. The quoted direction for the apparent Moon-Jupiter conjunction is then found to be accurate to within 1°. This delay of the apparent conjunction largely explains the error of 29° in their longitude determination. The truthfulness of the observations brackening the first recording of the NZ effect, debated for four centuries, now appears to be beyond doubt.
Published in Icarus 102, 62-75, doi:10.1006/icar.1993.1032 in 1993
G. P. Können, A. A. Schoenmaker and J. Tinbergen
Scanning the polarization of Venus at scattering angles 18-32° and at wavelengths 402-850 nm, we found a dip in polarization in the scattering angle range 23-25° for wavelengths 622 nm and longer. The width of the dip was 1-3°, its magnitude 0.4% in degree of polarization. The dip is consistent with the occurrence of a halo in the Venus atmosphere due to H2SO4-contaminated ice crystals in the upper haze layer of Venus. It remains unclear however why the halo is manifesting itself only at long wavelengths.
Published in Gemini 20, 12-13 in 1988
G. P. Können and J. Tinbergen
An account is given about our attempts at La Palma Observatory to detect ice crystals in the Venus's atmosphere, including the drawback of a temporary instrumental malfunction and our subsequent fortune that we got the opportunity to scan a terrestrial halo in a Venus-spoiling cirrus deck.
Published in J. Opt. Soc. Am. A 4, 601-608, doi:10.1364/JOSAA.4.000601 in 1987
G. P. Können
The polarization distribution in the sky during a total solar eclipse is calculated with a simple secondary light scattering model. The model can explain various observations during totality, including the measurements by Shaw of the polarization distribution of the sky in the solar vertical during the 1973 total eclipse.
Published in J. Brit. Astron. Assoc. 93, 20-24 in 1982
G. P. Können and J. Meeus
Triple planet-planet conjunctions are presented for AD 0-3000. It is shown that for Mars-Jupiter and Mars-Saturn two subsequent triple conjunctions can occur with a time separation of one synodic period. Triple conjunctions of bright planets with 1st magnitude stars are briefly discussed.
Published in J. Brit. Astron. Assoc. 91, 148-157 in 1981
G. P. Können and J. van Maanen
All occultations of stars brighter than 3.52 by the five bright planets have been calculated for AD 1900-2100; for the first magnitude stars for 1000-3000. In the 21st century, there is a Venus-Regulus, a Venus-p Sgr, a Mercury-aLib and a Mars-qOph occultation.
Published in J. Brit. Astron. Assoc. 87, 135-145 in 1977
J. Meeus, J. van Maanen and G. P. Können
We calculated these events for the bright outer planets for -100 till +3000 AD. No cases are founf before the 24th century. For Uranus 1850-2050 there are 4 cases, two of them in the 20th century. The rarity of the events in the present time can be explained by the fact that they occur in clusters separated by long-term periodicities.
Published in J. Roy. Astron. Soc. Canada 70, 81-83 in 1976
G. P. Können and J. Meeus
The rule of Van den Bergh that any time interval between two eclipses can be expressed as a linear combination of integer number of Saros plus an integer number of Inex is supplemented to avoid trivial outcomes.
Published in Weather 31, 425-426 in 1976
J.P. de Jongh and G. P. Können
The shadow of the annual eclipse of 29 April 1976 stands clearly out against the Sahara on the picture taken on 09:37 UT by the NOOA-4 weather satellite.
Published in Weather 30, 372-373 in 1975
G. P. Können and B. Zwart
A bright subsun was spotted on the image taken by the ESSA-8 weather satellite. The subsun appeared on a frontal system east of Greenland. To our knowledge this is the first time that a halo is identified on satellite picture of the Earth.
Published in J. Brit. Astron. Assoc. 85, 17-24 in 1974
G. P. Können and J. Meeus
11 stars brighter than magnitude 4 can be occulted by the totally eclipsed Moon, among them one (Regulus) is of the first magnitude. 7 additional stars can be occulted by bright site of the partially eclipsed Moon. List are given for events between 1900 and 2050. For Regulus, events are calculated for 0-2450. Events are found to occur in clusters. The last Regulus event was in 1943, the next is in 2445. Periodicities are discussed.
Published in J. Brit. Astron. Assoc. 82, 431-433 in 1972
G. P. Können and J. Meeus
The occultations by the Moon of Aldebaran, Regulus, Spica, Antares and Alcyone occuring between 1940 and 2050 are calculated.
Published in J. Brit. Astron. Assoc. 82, 192-193 in 1972
G. P. Können and J. Meeus
The extreme geocentric declinations of the Moon are presented for 1920-2050.