I'm not entirely sure which sub-forum this subject best fits. This claim does sometimes crop up in youtube videos and comments, but it's a more generally held psuedoscientific belief that can be found on a variety of conspiracy forums across the internet. Here are just a few examples to illustrate how widespread the belief is that Venus is now brighter than normal:
http://www.davidicke.com/forum/showthread.php?t=51432
http://www.godlikeproductions.com/fo...age1850362/pg1
http://www.abovetopsecret.com/forum/thread787463/pg5
Of course Venus' apparent magnitude varies as a function of its phase angle and distance from the earth and sun, but many seem to believe that it's brighter than it should be or has been historically. Most are going based on their own personal memory, which is both fallible and prone to bias. I have seen some debunkers use old photos of Venus to show that it has indeed been quite bright in decades past. Old photographs of Venus can be interesting in an anecdotal sense, but in and of themselves do not typically provide hard data on the apparent magnitude of Venus; film is notoriously non-linear in its response to light. In order to definitively determine if the brightness of Venus is normal compared to its historical brightness or not, two things are necessary; accurate, quantitative measurement of the current brightness of Venus (photometry) and a way of comparing this measurement to an expected value based solely on historical data (Danjon's formula).
In astronomy, an object's apparent brightness is measured in terms of apparent magnitude and can be determined using the following formula:
m(x)=-2.5*Log10(F(x)/F(x,0))+m(x,0)
where
m(x) = magnitude of unknown
m(x,0) = reference magnitude
F(x) = flux of unknown
F(x,0) = reference flux
Human estimates of an object's magnitude are lower in accuracy than they are in precision; the human eye can fairly accurately detect changes in brightness while monitoring an object like a variable star, but observer to observer variations can be quite high. On page 4 of the book "Astronomical Photometry: Past, Present, and Future" within the article titled "Photometric Precision and Accuracy" by Christiaan Sterken, E.F. Milone, and Andrew T. Young, it states the following:
"Although the visual estimates mimic changes of the variable quite closely, they demonstrate significant systematic zero-point deviations - in other words, they have good precision, but very poor accuracy. In particular, the estimates obtained by the two visual observers differ by 0.2m to 0.5m in 2007-2008."
Figure 1 of this article shows the precision of naked eye measurements to be no better than .1 magnitude, so because these anecdotal reports of Venus being "too bright" are themselves naked eye "measurements" (generally by untrained observers, no less), that is the minimum accuracy I aimed to achieve both with my own quantitative measurement of Venus' magnitude as well as with the method by which I compared this to its historically expected value.
I used CCD photometry to measure the brightness of Venus on the morning of September 17th, 2012. On that date, the star Sirius had about the same altitude over the horizon as Venus, which simplified the measurement by allowing for a direct comparison (since they would both experience similar amounts of atmospheric extinction). Sirius served as my reference flux, and since it has a known magnitude (and since it is bright enough to not be under-exposed in a fast exposure needed to avoid over-exposing Venus), it allowed me to solve for the magnitude of Venus.
At 05:38 UT on September 17th, I imaged both Venus and Sirius back to back using a Planewave 20" Corrected Dall-Kirkham Astrograph with a FLI ProLine PL11002M CCD camera and a photometric V filter, the T-11 telescope on the itelescope.net network. Venus had a measured intensity of 118458.254726 and Sirius was 10586.866282. These are arbitrary units, so we need to plug them into the above formula to calculate the magnitude of Venus. Given that Sirius has a photometric V magnitude of -1.47 (http://simbad.u-strasbg.fr/simbad/si...IMBAD%20search ), Venus had a V magnitude of about -4.09 that morning.
To compare this to the historically expected magnitude of Venus, I utilized the Danjon formula. In 1949, Andre-Louis Danjon published a formula for calculating the variation of Venus' visual magnitude (V) based on the solar phase angle using data he had collected over the previous 10 years.
http://i319.photobucket.com/albums/m...agnitudes2.jpg
Plugging the result into the general formula for calculating the magnitude of a planet, seen here (http://i319.photobucket.com/albums/m...agnitudes1.jpg ) allows one to calculate the expected magnitude of Venus given data from the first half of the 20th century, long before Venus supposedly started becoming "abnormally bright." Danjon's formula agrees with later determinations of Venus' magnitude (Knuckles, CF., Sinton, MK., and Sinton, WM 1961, "UBV Photometry Venus," Lowell Obs. Bull., 5, 153-156) to within about .1 magnitudes, the accuracy I'm aiming for. Given the distance of Venus from the sun and earth at that time (0.721802 and 0.957722 AUs respectively) and a phase angle of 71.9646 degrees, Venus' apparent V magnitude should have been about -4.03, agreeing with the measured value above well within the accuracy expected for the Danjon formula, and far beyond the accuracy and precision attainable by naked eye visual measurements.
I summarize all of this in the following YouTube video and also show the analysis of the images I took of Venus and Sirius (I used the astronomy version of ImageJ):
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Originally Posted by FreiZeitGeist
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