Abstract:
Daylight, naked-eye observations of the star Arcturus are presented. The author speculates on the role of very low atmospheric humidity in making daytime sightings of bright stars possible. We invite others in dry climates to attempt similar observations. We hope to open the arena of daylight star observation to more curiosity.
On July 6, 2020 in Tucson, Arizona, in the late afternoon, perhaps around 5:00 pm, my friend Jeanette Adrian and I happened to observe a star in the blue, daytime sky. Jeanette first noticed, and asked me what it was. It was a casual, naked-eye observation. She pointed to it, and I immediately spotted it.
I responded that it looked quite a bit like Venus when seen in a daytime sky, but being a person who regularly observes planets in the night sky, I happened to know this could not have been Venus. Venus was at the time west of the sun, rising in the morning sky, and at that time of the day, with the Sun approaching the western horizon, Venus was probably setting in the west just then.
This object was in the eastern half of the sky. If it was a celestial body, it was in the process of rising, and I would have guessed it to be approximately half way along its trip from the eastern horizon to the meridian. Along with ruling out Venus, I also knew that Jupiter was appearing low in the east not long after sunset. I quickly reasoned that this was not Jupiter, as this object was already much higher in the sky at 5 pm than Jupiter would be at 9 pm.
Without the possibility of it being Venus or Jupiter, I was baffled. I have been a regular sky observer since my teen years. I am educated in physics and astronomy, and I have long had a basic conception that the only "star" visible in daytime is Venus. It just so happened I had recently read somewhere that Jupiter can also be viewed in the daytime.
Being baffled, Jeanette and I simply continued to observe and enjoy the star for perhaps another twenty minutes. I mused that the object might be man-made. Perhaps a drone was hovering very high in the sky. I was seeking any explanation, no matter how unlikely, as I thought it impossible to see actual stars in broad daylight. I briefly wondered if it was Sirius, the brightest star. However, I knew this object was simply not following the track Sirius follows across the sky, as seen from Tucson's latitude.
I resolved to check the sky the following day around the same time. So, on July 7th, around the same time, Jeanette and I finished a swim and faced the sky on towels in the same spot, looking east. And there was the object again! Now I strongly suspected it was a star. I have never installed any sort of sky navigation guide on my phone, so I knew I would simply have to look at the sky after dark, and find out which bright star was in the vicinity. I estimated where the object would end up three hours later if it displayed diurnal motion.
When darkness fell, I stepped out and looked up at the familiar sight of the early evening summer sky. There was Arcturus, prominently near the zenith. I immediately knew Arcturus was the object we had spotted in broad daylight.
As an aside, our view was perfectly directed toward Arcturus by the surrounding walls, roofs and trees. In this image, Arcturus was centered near the top of this view. This natural guidance helped in easily finding it again the next day.
The reason I've written up this story in full detail is that the observation seems strange. Arcturus was not difficult to see in daylight. It was right there, plain as day, quite easy to pick out of the blue sky. How would I have never heard of Arcturus being visible in the daytime, if it was that easy to spot? Arcturus is not even the brightest star in the sky. Why haven't I heard of Sirius being observed in the day time?
As of now, I do not have an explanation. Naturally, I have sought background information on the Internet. On Wikipedia, one will find that "Jean-Baptiste Morin observed Arcturus in the daytime with a telescope in 1635." Further, there are mentions of Sirius and Arcturus being viewed for a few minutes before sunset, or after sunrise. I was unaware of that type of observation. That makes sense because when the sun is just barely above the horizon, the sky is slightly darker than the usual daytime sky. There is also an old legend that daytime stars are visible from deep down in a well. Numerous commentators have debunked the notion.
Much more could be said on the issue. I did not intend to write a formal paper on the matter. I simply wanted to convey the basics of the situation, and my understanding is that astronomers have never considered stars to be visible to the human eye when the sun is high in the sky, except for the rare situation of a total (or nearly total) solar eclipse. Despite this, Jeanette and I easily viewed Arcturus two afternoons in a row in the Tucson sky. On July 8th another friend and fellow Tucsonan, Dr. Erin Galyen, joined me in viewing Arcturus. Erin has the same degree in astronomy that I have (MSci from San Diego State), and she has done research in astronomy education at the University of Arizona. Erin found the observation just as surprising as I did.
Therefore, I am inviting others to consider the issue, and to attempt to make the same observation. It will be interesting to see if it can be replicated elsewhere. I have surmised that the very dry air of a Southern Arizona summer might be the key factor. Perhaps Arcturus is only visible when it rises fairly high in the sky, on a very hot day with very dry air. Those conditions are not met on most of the surface of the earth most of the time.
...
I wrote the above on July 9th, 2020. On that day I did not see Arcturus. Now, almost forty days later, I still have not seen it again in the daytime sky. I did not look every day, nor did I take detailed notes. Most days that were not too cloudy, I did spend a minute or two staring into the blue sky (usually around 5:00pm), hoping for another sighting.
In the intervening time, I have found a web site providing humidity data for Tucson (www.wunderground.com). Having looked at humidity records, I believe my initial theory probably has merit. The late afternoon humidity on July 6th was 8%, then 9% on the 7th, and about 12 to 13% on the 8th.
I have not made a formal study of Tucson humidity (nor do I think it is needed at this point), but I have looked at numerous historical humidity readings for Tucson late afternoons. It appears it is quite rare for the humidity to get below about 13 or 14%, and that appears to be the demarcation for our observations of Arcturus. Since July 8th, the 5pm humidity has been typically in the 15 to 20% range, and sometimes much higher.
Further, among the three sightings, it seemed quite a bit harder to spot on July 8th, the most humid day. The easiest observation was definitely on the 6th, with the lowest humidity.
I posit that very low humidity, under about 13%, can lead to daytime visibility for the brightest stars. The star must also be situated far enough from the Sun to avoid washout, and far enough above the horizon to avoid atmospheric dimming. These conditions severely limit how often a candidate star will even be available to test the theory. It might only happen a handful of days per year. However, if enough people learn of this idea, we should be able to get confirmation relatively soon. Please feel free to contact the author with your own personal reports.
From mid-northern latitudes, Sirius might not ever reach high enough altitudes. However, the theory could be tested with Sirius from deserts in the Southern hemisphere. The same goes for Canopus and Alpha Centauri. However, fewer observers live in the Southern hemisphere, which might explain why such an observation hasn't yet been claimed, even though the Southern sky is richer in bright stars.
I have no speculation yet on what the limiting magnitude of a star might be to be a candidate for this kind of daytime observation. Vega, Capella, and Rigel are only slightly less bright than Arcturus, and given how easily we saw Arcturus, these other Northern Sky stars might well also become visible on rare occasions.
(Note that the red color of Arcturus could conceivably play a role in its visibility. A more white or blue star may present less contrast with the blue sky. If that is the case, perhaps Arcturus is a better candidate for this type of observation than Sirius.)
Information about daytime star observation seems scant. Perhaps others have documented more. I will attempt further research. In the meantime, I thought the observation interesting enough to publicize. With modern technology available allowing observers to quickly ascertain their local humidity reading, as well as maps of celestial objects in their daytime sky, perhaps the elusive observation could become a desired "bucket list" experience among the world astronomy community.
The Tucson sky is rather magical. In my 17 years here, I've come to see that the daytime sky does have quite a different appearance than skies in less arid places. Here one often sees shadows of small clouds extending in long streaks, and great variations in the shades of blue, seen at one time in different spots in the sky. Those variations probably have to do with patches of varying humidity. On very dry days, the blackness of space seems to be close to being revealed behind the blue. I never imagined seeing Arcturus in the daytime, but now that I have, it seems to make sense that the discovery would happen here.
Jon Dokter
Tucson, Arizona
August 17, 2020
Footnotes:
Dr. Erin Galyen has a history of authorship in academic journals and conference proceedings as Erin Dokter.
Jeanette Adrian is an astronomy aficianado in the spirit of Walt Whitman's When I Heard the Learn'd Astronomer.
Contact: Jon, entropyratings@gmail.com
It's not easy to ascertain from searching the Internet what exactly is known about daytime star observations. We acknowledge literature on the matter might well exist of which we are unaware. Ours seemed like interesting observations, and perhaps unique. I shall continue to investigate, and relevant updates will be posted.
Appendix:
I'm going to add some Tucson humidity stats. Check again later if you want to see!