Issue 104

Equinox Sky Camp—
Trips / Events
Ideas for trips and events
always welcome!
[email protected]
 4 Aug WAS—Dark
Future—Bob Mizon
 15 July CADAS—Ask the
Experts Evening
 18 July BNSS—Sir John
Herschel – Astronomer by
Inheritance—Dr Allan
Chapman
 11 Aug BNSS—
Communication Satellites,
the History- Alan Jefferis
 19includes
Aug CADAS—Bob’s
This
free evePlanetarium
ning viewing. Doors
 1 Sept WAS—The Sky’s
Dark Labyrinth—Stuart
Clark
 1 Sept BNSS—Global
Nuclear Energy for 10,000
years—Brendan McNamara
 16 Sept CADAS—Rocks
from Space—Ron Westmaas
 26 Sept BNSS—Dark
Skies—Bob Mizon
 6 Oct WAS—AGM &
Astronomers’ Question Time
 21 Oct CADAS—APOD
Evening—Bob Mizon
More to come!
WAC Upcoming Events:
14 August—Public Open Evening
11 Sept—Eclipses I have
known—Chris Bowden
9 Oct—Auroras on Earth and
Beyond—Sheri Karl
13 Nov—Exoplanets—Gemma
Lavender
11 Dec—Christmas Quiz Night
More to come!
Plans for informal viewing nights
will take place after the monthly
meetings, weather permitting.
Sky Watcher
Volume 10, Issue 3
10 July 2015
WAC News—
This month Ron Westmaas from CADAS
sent in a stunning close up of AR2371 taken with
a Baader filter on his Celestron and a Solar
continuum and IR filter on a DMK camera.
Amazing what detail an amateur setup can
produce today. Well done Ron!
There have been numerous images of Noctilucent
clouds this past month. Keep a watch during the
summer when the sun is 15 degrees below the
horizon.
More interesting info at http://
w ww. nights kyhunt er.c om/ Noc tiluc ent %
20Clouds.html.
Errata: In last month’s newsletter there was an error in the formulae which made it into print
from Paul Masham’s fascinating article. Please let Paul or myself know if you would like the full
version of the correct article. Until next month...clear skies! ~SK
No Surprise! Earth’s Strongest Gravity Lies Atop The Highest Mountains
By Dr. Ethan Siegel
Put more mass beneath your
feet and feel the downward
acceleration due to gravity
increase. Newton's law of
universal gravitation may have been
superseded by Einstein's, but it still
describes the gravitational force and
acceleration here on Earth to remarkable
precision. The acceleration you experience
is directly proportional to the amount of
mass you "see," but inversely proportional
to the distance from you to that mass
squared. The denser the mass beneath
your feet, the stronger the gravitational
force, and when you are closer to such a
mass, the force is even greater. At higher
elevations or even higher altitudes, you'd
expect your gravitational force to drop as
you move farther from Earth's center.
You'd probably also expect that downward
acceleration to be greater if you stood atop
a large mountain than if you flew tens of
thousands of feet above a flat ocean, with
nothing but ultra-light air and liquid water
beneath you for all those
miles. In fact this is true, but
not just due to the mountain’s
extra mass! Earth is built like a
layer-cake, with the less dense
atmosphere, ocean, and crust
floating atop the denser
mantle, which in turn floats
atop the outer and inner cores
of our planet. An iceberg’s
buoyancy is enough to lift only
about one tenth of it above the
sea, with the other nine tenths
below the surface. Similarly,
each and every mountain range has a
corresponding "invisible mountain" that
dips deep into the mantle. Beneath the
ocean floor, Earth's crust might be only
three to six miles thick, but it can exceed
40 miles in thickness around major
mountain ranges like the Himalayas and
the Andes. It’s where one of Earth’s
tectonic plates subducts beneath another
that we see the largest gravitational
anomalies: another confirmation of the
theory of continental drift.
A combination of instruments aboard
NASA's Gravity Recovery and Climate
Experiment (GRACE) satellites, including
the SuperSTAR accelerometer, the
K-band ranging system and the onboard
GPS receiver, have enabled the construction of the most accurate map of Earth's
gravitational field ever: to accelerations of
nanometers per second squared. While
the mountaintops may be farther from
Earth's center than any other point, the
extra mass of the mountains and their
www.weymouthastronomy.co.uk
Page 2
Sky Watcher
Volume 10, Issue 3
roots – minus the mass of the displaced mantle – accounts for the true gravitational
accelerations we actually see. It's only by the grace of these satellites that we can
measure this to such accuracy and confirm what was first conjectured in the 1800s:
that the full layer-cake structure of Earth must be accounted for to explain the gravity we experience on our world!
Gravity (continued)
Article of the Month—Seeing Straight by John Gifford
Aligning the elements of a telescope system, how
and why.
From where we stand the sky appears to make
one full turn around the Earth every day. The fact
that what we see is really the Earth turning
beneath the sky makes no difference to how it
looks. There are 360 degrees in a circle and 24
hours in a day. That’s 15 degrees an hour, 1
degree in four minutes. The width of the Sun or
Moon every 2 minutes. Take a time exposure with
EQAF`1
a fixed camera and very soon your stars are not
round, given time they become arcs. For the visual
observer this movement is an inconvenience, whatever one is looking at gently exits stage left. For the photographer it
is a disaster. So, the telescope and camera have to track the sky.
When it comes to tracking the sky for us amateurs the standard answer is the equatorial mount. You can do the same
things with a fork mounted telescope but it is a bit fiddlier to get right in my view. (EQAF 1) The aim is to get the polar
axis aimed at the point in the sky around which everything appears to rotate. If that is achieved then rotating the polar
axis westward at 15 degrees per hour will make the telescope track whatever it is pointed at across the sky.
The first thing to do is place the mount in the intended observing position with the polar axis pointed at the pole star as
near as you can judge. For most visual observing that will be good enough. If we wish to take long photographic
exposures we will need to be a little more precise. Fitting a polar scope to the mount makes this much easier.
The polar scope is a small telescope fitted inside the polar axis of the main mount or, if that is not possible exactly
parallel to that axis. It cannot be on the axis with a fork mount, one of the sources of difficulty. Upon looking through
the polar scope this is what you will see. The central cross should be pointed to the
pole. (EQAF2) Naturally Polaris is only near the pole, not exactly on it, that would be
EQAF`2
far too easy. The radius of the small central circle in the illustration is the apparent
distance of Polaris from the pole. Like everything else in the sky Polaris will appear to
complete one circuit of the pole every 24 Hours. Set the reticule so that the tiny circle
on the small circle is at the correct o’clock for Polaris at the time of observation. Do
not forget that the view in the polar scope is reversed so that 3 o’clock is nine o’clock,
eight o’clock is 2 o’clock and so on. The correct position for Polaris on that circle can
be found from planetarium software by zooming in on the current position of Polaris.
Alternatively some of the cleverer mounts will tell you where Polaris ought to be on
the circle once you have told them their location, the date and the time.
Once this is known the polar scope can be set to the correct o‘clock and the mount
adjusted using the vertical and horizontal adjustment screws. When Polaris is inside
the tiny circle the alignment will be within around 2 arc min of true. For most of us most of the time that will be good
enough. If that is not good enough then you should google drift alignment and prepare to be very patient.
This method only works if the mounts polar axis and the polar scope are truly concentric.
It is much easier to check and adjust this in daylight using a fixed distant object. First set the mount so that the polar
axis is horizontal. This will affect the balance of the mount on the tripod, you may need to weigh down or tether the
back of the mount to avoid it falling over.
With the polar axis pointed at the horizon choose a fixed object at least EQAF`3
a mile away (Further is better.) and place it in the centre of the polar
scope view. Chimney pots, power pylons and gable peaks are all good.
(EQAF3) Rotate the mount around the polar axis. If the object stays put
all is well, if not it will describe a circle touching your target at one point.
Adjust the alignment of the polar scope using the three tiny screws near
the eyepiece end until concentricity is achieved. (EQAF3 inset)
This may take a while. The first time that you do this it will take ages
and cause much cursing but it is worth it. While you have this setup you
could usefully use the same technique to get your main telescope and
the finder scope lined up with the mount too.
www.weymouthastronomy.co.uk