The meteor is the streak diagonally across the upper right of the photograph, the direction of the meteor's travel being from lower-right (from Leo) upwards. Strangely enough, another photograph taken in a different five-minute time slot, with the camera
unmoved on its tripod, captured the image of another meteor in almost exactly the same trajectory.
Meteors are named after the part of the sky (the constellation) from which the meteors appear to have
come. For example, if all Leonid meteor tracks were plotted backwards, it is in
the region in the sky of the constellation of Leo where they would appear to
originate.
The earth, during its yearly orbit around the sun, intercepts these dust trails of material shed by
comets and in doing so accumulates huge amounts of celestial material. It is the
heat from the friction of the particle's passage through the earth's atmosphere
that causes the meteor to glow. They generally burn up in the upper layers of
the atmosphere. Some, often car sized rocky or metallic chunks. tend to break-up
in their fiery plunge, becoming "fireballs" or "bolides",
like that which was seen over the lower North Island of NZ in July 1999, and may
give off a sonic boom. It is only the larger ones that survive to hit the
earth's surface. These "survivors" are known as "meteorites".
Meteors are generally sand size specks of dust, rock, or ice left behind from the trail of a comet,
but they can be bigger and sometimes are. Meteors can also originate
from the "Meteor Belt" between Mars and Jupiter and also from the
"Oort Cloud" way out beyond Pluto.
Some meteors in the pre-historic past have hit the earth with catastrophic results, e.g the impact
about 64 million years ago in the region of Yucatan Peninsula of a meteorite
(whose diameter was in the region of 10km) which caused world-wide devastation
and a "Nuclear Winter" type scenario that resulted in the extinction of
about 99% of all animal life on Earth, notably ending the reign of the
dinosaurs. Another example is the 80 metre wide meteorite which impacted
in Arizona about 27,000 years ago and created the famous Arizona Crater which is
in the region of about 1,265 metres wide and about 175 metres deep.
Periodically large objects do hit the earth. The larger they are, the longer the mean
interval between them. Based on average time intervals between impacts, the
Earth is due for another hit by a large "impactor". However,
don't get too alarmed at this. We are talking of average time intervals
not in years or hundreds or even thousands of years, but in millions of
years. However, that impact "could" happen tomorrow — so
enjoy today.
Partial Solar Eclipse
|
|
|
|
|
|
|
|
|
|
Progress of Partial Solar Eclipse August 22nd 1998 © Alan Torrance 200mm focal length lens. Cloud partially obscured some of the observations above, and then totally. Viewed from Waharau Outdoor Activities Centre, as part of an Auckland Astronomical Society "Dark Sky" Weekend. |
||||||||
Making My Own Telescope
I am currently in the process of making my own telescope. This will be one of the
"Newtonian" variety, i.e. the light from a primary mirror is reflected
to impinge on a smaller flat mirror held at 45 degrees which in turn reflects
the incoming light to the eyepiece on the side of the telescope tube. [See
simple diagram "Newtonian" in "Telescope Basics"].
I have ground my primary mirror by hand, using traditional hand grinding methods, but have not yet polished it.
It has yet to be configured from its present spherical shape to that of a parabola to permit proper focussing of the
light. Its diameter is 30cm and has a short focal length giving it an "f"
ratio of only 3.8. It will be a very compact instrument when completed —
suitable for "deep sky" work, rather than planetary.
Unfortunately, due to the short focal length, it will have a degree of
"coma", a type of focal aberration affecting the outer sectors of the
focal field. However..... as my attention will be more on the centre, this
should not trouble me too much.
The "flat" mirror, the "spider", the "focuser", as well as the various
other bits of hardware holding the mirror have all yet to be constructed.
The "tube" will be constructed of thin plywood wrapped around a series of thicker plywood
hoops which are themselves held in position by a series of struts.
I have not yet decided what form the mount will take. This may be the simple
"Dobsonian" or the more elaborate "Equatorial".