Two questions: 1) Is the colour of a planet's sky dependent on the colour of the planet's sun, given that its atmosphere is Earth-like? To put it another way, if you had the power to change both the Sun's luminosity and spectral type, but nothing else, could you turn Earth's sky red or green, or would brighter blues on the one hand and a night-time-like sky on the other be the extents of what you could achieve? 2) How bright does a celestial object have to be in order to be visible right next to the Sun? Is it sufficient to be brighter than the day-time sky, or do I have to add an extra margin for "solar glare" or some such? Since none of the familiar celestial objects clearly qualify, historical accounts of supernovae might be a good place to look, thought I. After a bit of digging and figuring, it turns out that the famous Crab supernova, aka SN 1054, was indeed fairly close to the Sun during its maximum: It occurred in early July at about RA 5h 30' Dec +22°. At the summer solstice, i.e. just a week earlier, the Sun is at exactly RA 6h Dec +23.5° (this follows directly from the definition of the celestial coordinate system, unless I messed up). And voila, I found this passage in a related paper: In order to estimate m_i [the limiting apparent magnitude of an object that can be seen with the unaided eye in daylight], we make general reference to the literature of planetary observations, in which there frequently occurs the statement that Venus may be seen in daylight with the unaided eye at nearly all points of its orbit. [...] Since the apparent magnitude of Venus, in this instance, was -3.3, it seems reasonable to infer that an object of -3.5 vis. mag. could readily been [sic] seen farther from the sun, especially if the observer knew just where to look. In the case of the Chinese observations, the nova was discovered close to Zeta Tauri in the early morning sky, at a time (July 4) when the Sun was more than two hours east of the nova. When the latter could no longer be seen in daylight [end of July], the sun was nearly four hours east. Under these circumstances, the Chinese probably knew precisely where to look for their guest-star in daytime, and may have been able to follow it in daylight to an apparent magnitude of -3.5, which we shall therefore take as m_i. Now, this is helpful but not quite what I had in mind. The limit the article addresses is what one is able to discern if one tries really hard. The limit I'm wondering about is what one can't help but see if one happens to look in the right direction. Venus certainly meets that criterion just before sunrise and just after sunset, but it just as certainly doesn't meet it at noon. If there were such a thing as a full moon close to the Sun, it might qualify; I'm not sure. Any help? edited 27th Dec '10 3:16:51 AM by kassyopeia
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