Tuesday, 5 August 2008

IAA Solar Day, Perseids, Lunar Eclipse, Sky at Night

1. IAA SOLAR DAY. The first of this year's popular IAA 'Solar Days' will be
held at Carnfunnock Country Park on Saturday 9 August, from 2 to 5 p.m. As
usual, we'll have a good selection of solar telescopes & ordinary telescopes
with special solar filters and/or Sun projection systems. Spots have been rare
recently, but the solar telescopes will also show prominences on the limb, and
faculae and plages on the disc. Carnfunnock Country Park also has the best
collection of sundials in Ireland, all of different types. We'll have
alternative entertainment if it's cloudy, so come along for some spectacular
views of our nearest star.

2. PERSEID METEORS. The 'Old Faithful' of meteor showers is with us once
again, now building up to the maximum rate expected on August 12. Some Perseid
meteors are already appearing, but the rate does not reach a really noticeable
level until about August 7/8.

The peak is forecast for about 09h on the 12th, so best rates
will be seen on the night of 11/12, just before dawn starts to lighten the sky.
The waxing gibbous Moon will interfere somewhat on the night of
maximum, with First Quarter being on the evening of the 8th, and Full Moon on
the evening of the 16th. But the Moon will set by about 00.30 that
night, giving a reasonable dark-sky window before dawn for keen observers.

The radiant, from where the meteors appear to originate, is in
NE Perseus, not far from the famous Double Cluster. It is above the horizon all
night from Irish/British latitudes, so some meteors can be seen as soon as the
sky gets dark enough - say by about 11.30 p.m.

The maximum ZHR is forecast to be about 80, but
please note that you won't actually see that rate!
Why? The ZHR, or Zenithal Hourly Rate, is defined as the rate that would be seen
by an experienced observer, in a very dark sky, with the radiant in the
zenith. Even though the Moon will set well before dawn, and even if you
observe from a very dark location, there's one insuperable problem - the radiant
does not reach the zenith before dawn! From Belfast, for example, it only
reaches an altitude of about 65 degrees before the sky starts to brighten. So
don't believe so-called experts and commentators who just blindly quote the
official ZHR as the rate that the average person will see, ignoring all the
other factors! I even heard one professional astronomer say that the ZHR rate
could be seen by anybody, not just on the night of maximum, but during the whole
2 weeks or so that the shower is officially 'active'!

However, 65 degrees is still quite good, so if all other
conditions are favourable, you might see 60 to 70 Perseids per hour during that
moonless window from a very dark site. Add in about another 10 sporadic meteors
per hour, and it starts to become quite a respectable show.

Rates drop off during the few days after maximum, and with the Moon
brightening to Full on the 16th, you won't see so many on the nights after
maximum. - But see below.....

3. ECLIPSE OF THE MOON, 16 August: Many people in Ireland saw the partial
eclipse of the Sun on 1 August. I watched it through filtered binoculars from
the office, and colleagues there were quite impressed with the view just with
binocs! Many IAA members got nice photos: see www.irishastro.org.

Well, that was New Moon, and at the next Full Moon, on 16
August, there's also an eclipse - a lunar one this time, of course. It also will
be a partial eclipse, but with a much greater magnitude that the solar eclipse.
At maximum, almost 81% of the Moon will be in the Earth's shadow, and the rest
will be dimmed by the outer fainter penumbral shadow. From Ireland, the Moon
will rise already partially eclipsed, at about 21.45 from Belfast & Dublin,
and a bit later the further West you go.

Maximum eclipse will occur at 22.10 BST. The Moon's
altitude then will be less than 10 degrees, so you'll need a good clear SE
horizon to see the eclipse properly.

The Moon will pass through the NNW part of the Earth's shadow,
so it will be the SSE part of the Moon that will be deepest in the eclipse. The
moon leaves the umbra, or the main part of the Earth's shadow, at

Eclipse Perseids? Although the eclipse won't be
total, the Moon will be dimmed enough for the 15-20 minutes or so centred on
maximum eclipse for us to be able to spot some late Perseids if we are

4. The SKY AT NIGHT: Title: "Double Vision" (The Large Binocular
Telescope). If you missed the first broadcast on the 3/4 August, it
will be shown again as follows (thanks to Peter Paice for the info):
BBC 4 8 pm. Tues. 5 Aug.

Repeat: BBC 2 Sat. Aug. 9.

Clear Skies,

Terry Moseley

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