When we were in 3DA0, there was something else we did not
do! we missed the Perseids. I usually have one or two skeds
running but not in 2009.)
When we were in Hawane, Jury and Tony told me on different
occasions that they had seen shooting stars. I noticed a
nice one on the morning when I got up before sunrise.
It was in the north quite low in the sky. Since I 've
been back I've been thinking about this. For a start, as
far as I know, Perseus, the radiant point for these meteors,
is not visible in the Southern sky? So how could we see
Perseids in Swaziland. Here is a summary of my thinking
so far:
1) Meteors streak across different parts of the sky, but
for any particular shower, if you trace back their paths
they appear to originate from one point in the night sky.
2) For the Perseids, this is in the constellation Perseus.
(So far so good, and I think the northern hemisphere people
know this already.)
3) According to my trusty Planisphere (which is an old
plastic one and which has a useful working zone of latitude
46 - 56 degrees N) and if we take 0100 utc on 13 August
as a reference time & date, then Perseus is quite high,
not overhead but a bit to the south. (My Planisphere does
give the zenith angle, just azimuth). In other words, if
you are in the UK and lying on your back at that time, you
should see Perseids coming from that part of the sky, some
high in the sky and others lower down. Again we know this
because we have seen them!!
4) Now, what about an observer in the southern hemisphere?
Can he see anything? Looking on the web, I found (yet another!!)
very useful NASA
Website and put in some rough co-ords
26 S and 31 E (near enough to the part of the world we were
in.) and 56N 4 W (Scotland)
5) The calculator showed a peak of 4.5 per hour at 0530
utc for the African location and a broader peak and ten
times the rate (45 max) 2200 utc - 0300 utc for Scotland.
The time offset is not surprising and corresponds to the
longitudinal time difference. So far so good.
6) What is interesting is that the NASA model predicts
it is possible to "see" the Perseids at the southern location
just before sunrise.
7) How about the meteors we actually saw in Swazi ? The
sun did not rise until around 6 am (local time), so the
sighting would be sometime before 0400 utc, which fits!!
8) Then I wondered how far south you would have to be before
you would be "below the horizon" and not see the meteors,
so I put various numbers into the calculator on the NASA
website, and, at 40 degrees south, you cannot see the Perseids.
So, for example, someone in Cape Town wouldn't have been
able to observe what we saw in Hawane. We were just in range
for a sighting. We were lucky!
9) I'm still trying to think what exactly determines the
cut off point. The tilt of the earth's axis and the azimuthally
angle of the peak radiant would both seem to be relevant
- anyone good at solid trigonometry?
10) Famously, meteorites have been discovered on/in the
Antarctic ice, so presumably they come from showers whose
radiant is above the southern latitudes?
11) From a radio point of view, this seems to imply that
there are some meteor showers that we (in the North) cannot
use for scattering but are "available" to hams in countries
like South Africa at lower southern latitudes. Finally,
when I read this through, it all seems fairly obvious. Maybe
it has taken me 50 years (and a trip South of the equator!)
to get around to thinking about it a bit more deeply. Have
I got this right? Any thoughts from our friends in South
Africa? Do you have a particular meteor shower that comes
from out of the Southern sky?
73 Malcolm
