NASA’s Jet Propulsion Laboratory provides a useful service each month in publishing a “What’s Up” video to tell the public what they can see in the sky. It is generally good, so I was surprised by some misleading information in the December 2017 edition, in particular about observing meteor showers.
The video, narrated by JPL’s Jane Houston Jones, rightly promotes the Geminids as the best meteor shower of the year. It then tells viewers to expect to see up to 120 meteors an hour from a dark sky.
In fact, the figure of 120 an hour is the Zenithal Hourly Rate (ZHR) predicted for the Geminids and that number depends on a number of factors, including perfectly clear dark skies, and having the radiant overhead. For most of the world, it will never reach the zenith.
Irritatingly, the video graphic shows meteors pouring out of the radiant, close to the bright star Castor, and all clustered very close to the radiant. The truth is that the meteors will appear in all parts of the sky. If you trace their flight paths backwards, they will meet at the radiant.
You might put this down to a stylistic representation of the radiant, except that the error is compounded in the narration a little later, about a second meteor shower, as I shall explain a bit later on.
Sticking with the Geminids, the video tells us: “In the southern hemisphere, you won’t see as many, perhaps 10-20 per hour, because the radiant never rises above the horizon.” The accompanying animated graphic compounds this error, by illustrating a radiant a little below the horizon with fewer meteors flying above it.
There are two things wrong here. First of all, you won’t see any meteors from a shower while its radiant is significantly below the horizon. (You will still see non-shower meteors, or sporadics.) The shower’s meteoroids stream into the atmosphere in parallel paths, and from your own location, the Earth itself will be blocking them from reaching your sky before the radiant has risen.
However, the statement that the radiant never rises above the horizon in the southern hemisphere is simply not true. It will be the case for NASA scientists in the Antarctic, but they’ve got better reasons already not to see the Geminids because they’re bathed in midnight Sun in December!
Take a more typical centre of population in the southern hemisphere, such as Cape Town, South Africa, or Sydney, Australia, and you will find that the Geminids’ radiant most certainly does rise above the horizon, late in the night, and reaches an altitude of more than 20 degrees before dawn twilight. This is the reason why you can see Geminids from such typical southern latitudes, albeit in fewer number than from the northern hemisphere where the radiant gets much higher in the sky.
The video returns to meteors by mentioning the Ursids, a minor shower with its radiant in Ursa Minor, the Little Bear, which is also known in the USA as the Little Dipper.
Once again, we see the radiant with meteors all clustered close to it. I had put this down to being a stylistic representation, but then the narration says: “Have a look at the Little Dipper’s bowl, and you might see about ten meteors per hour.” Surely our NASA narrator doesn’t believe that meteors really do appear around the radiant?
As any seasoned meteor observer will advise, the radiant is not the place to look when you are observing a meteor shower. You will be much better off directing your gaze about 60 degrees away from it where you will catch meteors with longer paths. (Closer to the radiant, any that appear will appear to have very short tracks as they will be coming flying almost directly towards you!)
It is disappointing to see so many basic errors and misleading statements in one short video. I would hope that it might be pulled and reissued after some careful editing. Since that is unlikely to happen, may I instead suggest that more care is taken when writing about upcoming meteor showers in 2018?
NASA’s What’s Up video for December, which gets muddled over meteors.
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