Lowitz arcs - The 1790 St Petersburg Display

Last updated on September 16, 2023

Lowitz Arcs - The 1790 St. Petersburg Display

The phenomenon known as Lowitz arcs, observed by Tobias Lowitz in St. Petersburg on June 18, 1790, remains a fascinating topic in the field of atmospheric optics. Lasting for several hours, this display captured Lowitz's attention, leading him to document it through drawings. While his drawings comprised various rare arcs, the ones marked with red arrows garnered significant controversy and were subsequently named after him.

Lowitz's observations of the halos during the St. Petersburg display align quite well with the HaloSim4 simulations conducted for the peak of the event, which occurred around 10 am when the sun was approximately 51° high in the sky. Lowitz's drawings illustrate a sequence of halos, starting from the sun and moving outwards. These include a 22° halo, a circumscribed halo, and a 46° halo. Furthermore, Lowitz's depiction includes a complete parhelic circle with two 120° parhelia, along with two prominent infralateral arcs below the sun.

However, some aspects of Lowitz's drawings present intriguing puzzles. For instance, the lower sunvex arc tangent to the 22° halo raises questions about its origin and visibility. It could potentially be a lower tangent arc that was observed earlier when the sun was lower in the sky. Similarly, Lowitz's inclusion of a circumzenithal arc suggests that it could only have been seen earlier in the morning. These details add layers of complexity to our understanding of the St. Petersburg display.

While Lowitz's drawings exclusively depict the lower Lowitz arcs, it is unclear why he omitted the upper ones. One possibility is that he simply overlooked them during his observations. However, an alternative explanation arises from simulations using plate crystals with limited rotation. These simulations demonstrate that when plate crystals oscillate around their usual horizontal position with a Gaussian distribution of Lowitz tilts, the lower arc becomes relatively stronger near the parhelion, while the upper arc becomes weak and diffuse. In fact, the upper arc disappears entirely when the sun reaches an altitude of 50° or higher. If the plate crystals in St. Petersburg were similarly oriented, it is plausible that Lowitz would not have observed the upper arc during the event.

The study of atmospheric optics continues to shed light on the intricate and mesmerizing phenomena that occur in our skies. While Lowitz arcs remain a captivating subject, it is important to consider the limitations of historical observations and the potential influence of crystal orientations on the visibility of certain arcs. Exploring these complexities helps us refine our understanding of atmospheric phenomena and enhances our ability to interpret similar events in the future.

In conclusion, Lowitz arcs observed during the 1790 St. Petersburg display provide valuable insights into the world of atmospheric optics. Tobias Lowitz's meticulous drawings capture the beauty and complexity of this event, showcasing various rare arcs. While some aspects of his drawings raise intriguing questions, simulations using plate crystals with limited rotation offer potential explanations for the absence of upper Lowitz arcs in his observations. The study of atmospheric optics remains an ever-evolving field, allowing us to deepen our understanding of natural phenomena and appreciate the wonders that occur above us in the sky.

Tobias Lowitz observed this famous display at St. Petersburg June 18, 1790. It lasted for several hours and to some extent his drawing is a composite. Lowitz drew a number of rare arcs but none so controversial as those marked with red arrows. They extend downwards from each sundog to join the 22° halo and were subsequently named after him.

The non-Lowitz halos agree reasonably well with the all-sky HaloSim4 simulations made for the display's peak at ~10am when the sun was 51° high. Outwards from the sun Lowitz shows a 22° halo and circumscribed halo then a 46° halo. There is a complete parhelic circle with two 120° parhelia. Below the sun Lowitz shows two prominent infralateral arcs. The lower sunvex arc tangent to the 22 degree halo is puzzling. It could be a lower tangent arc seen earlier when the sun was lower. In the same sense, Lowitz shows a circumzenithal arc that could only have been seen earlier in the morning. Wegener arcs extend from the top of the circumscribed halo over the sky to the anthelion. Greenler's book has a very good account of the display.

Lowitz shows only the lower Lowitz arcs. Why? It is odd that his otherwise careful drawing omits the upper ones. Perhaps he missed them?

There is another explanation. The left hand simulation used plate crystals with classical Lowitz orientations - i.e. taking all rotational positions around the Lowitz axis. The upper Lowitz arcs, shown in red, are as prominent as the lower and both arcs are relatively weak near to the parhelia.

The right hand simulation used plate crystals with limited rotation. The plates were allowed to oscillate about their usual horizontal position with a Gaussian distribution of Lowitz tilts of standard deviation 20°. At fairly high sun two things then happen, (1) the lower arc becomes relatively stronger close to the parhelion and (2) the upper arc becomes weak and diffuse eventually dissappearing altogether when the sun is higher. If the plate crystals were similarly oriented at St. Petersburg, Lowitz would have been unlikely to see an upper arc when the sun was 50° high or more.

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