If you think that daytime sky observing is limited to clouds and bird-watching, you might be missing out. Observing space objects in the daytime has its limitations and difficulties, but, as with all skywatching, it also has its rewards. So here is a list of 10 surprising space objects to see in the daytime sky. Aside from the first three listed below, each of these daylight observations is relatively difficult. Plus some of these observations are not possible to predict. With all that said, here they are, in increasing order of difficulty: your top 10 space objects to see in daylight.
To keep this post short, I’ll leave lengthy discussion of actual observing for a later time, or will discuss in the comments. If you have a question or comment, please post.
Obviously, you can see the sun during the day, but paradoxically, we are told not to do it for fear of harming our eyes. And that is quite right. Gazing at the sun directly can damage your eyes. It comes as a surprise to some folks, then, that there are safe and relatively simple ways of observing the sun safely and inexpensively. In fact, I regularly assign an activity for students to project an image of the sun with no more than a mirror. This is the same kind of thing done whenever there is a solar eclipse or transit. How do you do it? Find out about a sun-observing technique called pinhole projection here.
By the way, the sun is the source of a whole range of atmospheric effects, which are beyond the scope of this post. To learn about things like rainbows and solar haloes, go to Les Cowley’s great website on atmospheric optics.
I don’t have any survey statistics, but I would be willing to bet that at least 75% of the public is unaware that the moon can be seen in the daytime sky. That’s not too hard to understand, since so many people nowadays spend so much time indoors and are unaware of the sky at all. In addition, the moon is not in the daytime sky every day. Like the sun, it is below the horizon half the time, such that about half the month it is in the daytime sky, and the other half in the night sky. Add that to the fact that much of the time the moon is up during the day, it is a thin crescent too close to the sun to be seen easily. It is easy to see why some people are surprised to discover the moon in the daytime sky. But voila. If you look up frequently, you’ll notice it often.
Anyone who is surprised that the moon can be seen in the daytime will be amazed that, under the right conditions, you can see the planet Venus with the sun also in the sky. In fact, many folks are surprised that planets can be seen with the unaided eye at all, much less during the daytime. However, anyone with good eyesight and a little patience can find Venus in the daytime sky, when Venus is well situated for this observation. The planet appears as a tiny white dot, which often seems to “pop” out at you once you find it. Anyone who has seen Venus in a reasonably dark sky knows that it is usually truly brilliant. Observations in the daytime sky are more difficult simply because the surrounding sky is so bright during the day. The contrast between planet and sky is much lower during the day, making the planet hard to see. Imagine how easy it is to see a bright light at the top of a tower at night versus daytime. That’s similar to seeing Venus at night versus day.
Many folks are very surprised that satellites can be seen at all, but these days they are quite common in dark, nighttime skies. Seasoned observers are more surprised when an hour of nighttime observing goes by without seeing at least one! They look like steadily moving “stars” – silent – and very high up. At least one type of satellite (Iridium) can sometimes be seen in the daytime sky, although this is uncommon. These communications satellites have very reflective surfaces and under the right conditions, can reflect enough sunlight to appear as bright dots moving across the sky for a few seconds. These flashes are known as Iridium flares.
Even some seasoned astronomers are surprised to learn that mighty Jupiter can be glimpsed with the unaided eye in a sunlit sky. I do not want to mislead you, as this is not an easy observation. Jupiter is significantly dimmer than Venus, and finding it takes a good bit more effort (not to mention exceptionally good eyesight and excellent atmospheric conditions). The best time is near a “quadrature” when Jupiter is about 90 degrees away from the sun in the sky. This is similar to the arrangement of first quarter and last quarter moon. In fact, it is also very helpful to have a quarter moon nearby as a kind of sky landmark to guide you to Jupiter. The reason you want the planet at about 90 degrees from the sun is that the sky is slightly darker there, due to a phenomenon known as polarization.
Only a relative few observers have caught Jupiter with the unaided eye the daytime, and even fewer have seen Mars. However, it is possible. On occasion Mars can be as bright as Jupiter (or perhaps a tiny tad brighter), and the suggestions for catching it are the same as for Jupiter. Although I personally have not seen Mars in the daytime sky (I’ve seen Jupiter twice), a correspondent in the Middle East has reported to me an apparently genuine observation, and I have no doubt that it can be done.
Stars can be seen in the daytime sky, but this is a bit of a cheat. Stars, along with the brighter planets already mentioned, can be seen with the unaided human eye in a daytime sky (that is, when the sun is above the horizon) normally only during a total solar eclipse. Such observations are of historical significance, and in fact played a crucial role in one of the first confirmations of Einstein’s theories of relativity. A few observers report that they have seen some bright stars, such as Sirius, with the unaided eye in the daytime sky. If indeed this is possible, it would require exceptional eyesight and exceptional sky conditions. On the other hand, observers with telescopes can see certain bright stars (not to mention the bright planets) on any clear day, although the scientific reasons for doing so are few and far between.
Like the meteors with which they are sometimes confused, bright comets have been documented in the daytime sky. In fact, although not necessarily easy to observe, they are not all that rare. Comet McNaught became visible in daylight skies in 2007, and a bright daytime comet preceded Halley’s Comet in 1910. Daytime comets are perhaps easier than daytime meteors because they sometimes can be predicted a short time ahead.
Rare and unpredictable, very bright meteors are sometimes seen in the daylight sky. One of the most famous incidents occurred over the western part of North America in 1972. It was seen and even filmed by observers from Utah to Alberta. The most recent (as of this writing) was reported over California and Nevada on April 22, 2012. This meteor streaked across the daylight sky, creating a sonic boom that rattled windows. It was seen by thousands. Later, astronomers said the meteor began as a mini-van-sized asteroid, and they located a debris field containing fragments of the meteorite, which is now known as the Sutter’s Mill meteorite.
Last on our list of space objects (sometimes) visible in the daytime sky are supernovae, or exploding stars. Estimates vary as to the expected frequency of supernovae explosions in our Milky Way galaxy from as many as once every 20 years to once every 300 years. We simply do not have enough records of these infrequent phenomena to give much of an average. Many of these would not even be visible from Earth due to intervening gas and dust. In any event, the last supernova bright enough to be seen in the daytime sky was in 1572, and then only barely. The most likely candidate for a supernova explosion visible during daytime is the star Betelgeuse. Unquestionably it will be visible in the day sky when it explodes, but when that will be is still unknown. It could be tonight, but more likely in a few thousand, or tens of thousands, or maybe even a million years from now.