Download Sky Notes - April 2012 - North Devon Astronomical Society

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Sky Notes - April 2012
THE MOON
New Moon 21st April
First Quarter 29th April
Full Moon 6th April
Last Quarter 13th April
THE PLANETS
Mercury
A morning object, Mercury is visible before sunrise by the middle of the month. The planet will reach
greatest western elongation on the 18th, when it will be around 27 degrees from the Sun.
Venus
Now moving closer to the Sun once again following greatest elongation last month, but blazing away at
magnitude -4.4, Venus still dominates the western evening sky. In the eyepiece, the planet presents a
25 arcsecond disc which is just under half illuminated.
Mars
Following last month’s opposition, The Red Planet remains fairly close to the bright star Regulus, in the
constellation Leo. Though the apparent diameter of the planet will shrink slightly throughout the
month, Mars remains worthy of observation and is visible for most of the night.
Jupiter
Though still visible low in the west after sunset, Jupiter will soon become lost in the evening twilight.
Saturn
Reaching opposition on the 15th, Saturn is observable all night in the constellation Virgo, close to the
first magnitude star Spica, (Alpha Virginis). A magnificent object, Saturn’s rings and it’s largest satellite
Titan are readily apparent in small telescopes, while larger instruments will show atmospheric bands
and some of the smaller moons.
Uranus
Uranus is too close to the Sun to be observable this month.
Neptune
A morning object, Neptune can be found among the stars of the constellation Aquarius.
APRIL HIGHLIGHTS
3rd
Venus passes very close to M45 - the Pleiades star cluster. This relatively rare event, (it won’t happen
again for another eight years), is sure to be both a stunning sight and a great imaging opportunity.
15th
Saturn at opposition - see above.
18th
Mercury at greatest western, (morning), elongation - see above.
21st
Peak of the Lyrid meteor shower. Caused by the dusty remnants of Comet C/1861 G1 Thatcher, the
Lyrids should be active between the 16th and 26th, and this year’s peak coincides with the new Moon
on the night of the 21st / 22nd. The shower radiant is located close to the bright star Vega in Lyra, but
best results should be obtained by not looking directly at this area. Peak rates are expected to be
around 15 meteors per hour at the zenith.
25th
On the evening of the 25th, the young crescent Moon will occult, (pass in front of), the magnitude +2.9
star Zeta ( .
) Tauri, in the constellation Taurus. The star will be seen to disappear behind the dark limb
of the Moon at 9.30pm, (as seen from London - Barnstaple will have to wait an extra few minutes), and
re-appear some 22 minutes later.
CEPHEUS
CONSTELLATION CLOSE-UP
This month, our target is the northern
circumpolar constellation Cepheus.
CAMELOPARDALIS
Another of the original 48 constellations drawn
up by Ptolemy in the 2nd century AD, Cepheus
represents the mythical king of Aetheopia,
(Etheopia) - husband of queen Cassiopeia and
father to Andromeda.
NGC 188
(
DRACO
Errai
$
Alfirk
,
CEPHEUS
M103
k
Though relatively unremarkable, as regards
deep sky targets for small telescopes, this
constellation contains several interesting
stars, including the prototype of an important
kind of variable.
Polaris
(
0
0
4
Alderamin
$ M52
Herschel’s Garnet Star
:
k
Covering some 588 square degrees and sitting
close to the Northern Celestial Pole, Cepheus
never sets, as seen from the UK, and is visible
throughout the year. Despite containing only
one star brighter than 3rd magnitude - the
magnitude +2.45 Alderamin, the constellation
is fairly easy to locate due to it’s proximity to
the pole star Polaris,.and the ‘W’ shape of
neighbouring Cassiopeia.
URSA MINOR
.
However, one interesting target for small to
CASSIOPEIA
LACERTA
moderate telescopes is NGC 188. This small
open cluster is the closest object of it’s type to
the Northern Celestial Pole and is one of the oldest open clusters known to astronomers.
.
CYGNUS
As mentioned above, Cepheus contains some interesting and important stars, some of which are listed here:
(
Cep Gamma Cephei, (also known as Errai), is a binary pair consisting of an orange sub-giant, orbited by a red
dwarf companion, and current estimates place the pair at a distance of around 50 light-years from us.
Interestingly, Gamma Cephei will be the pole star between 3000 and 5200 AD.
Cep Delta Cephei - the first Cepheid variable. Unlike eclipsing binaries, where the light from a star is dimmed by
the orbital motion of a smaller or less powerful companion, the variability in Cepheids is due to the
pulsation of the star itself. In the case of Delta Cephei, the star’s brightness ranges between magnitude +3.5
and +4.4 and occurs over a period of around five and a half days. Cepheids are important as their variability
is very regular, and this enables astronomers to use them as ‘standard candles’ to measure both galactic
and extra-galactic distances. Delta Cephei is around 887 light-years distant and has a magnitude +7.5
companion, (separation 41 arcseconds), which is visible in small telescopes.
:
Cep Popularly known as ‘Herschel’s Garnet Star’ due to it’s deep red colour, Mu Cephei is another variable star
whose luminosity ranges between magnitude +3.4 and +5.1 over a period of around 730 days. A red supergiant with a radius of around 11.8 astronomical units, this star is truly enormous. If the centre of Mu Cephei
were placed in the position of our Sun, it would reach out as far as the orbit of Saturn!
Cepheus is bordered by the constellations Draco, Cygnus, Lacerta, Cassiopeia, Camelopardalis and Ursa Minor.
Society meetings are held on the first Wednesday of each month at the Methodist Church Hall, Rhododendron Ave,
Sticklepath Hill, Barnstaple. Meetings begin at 7.45pm and all are welcome to attend, regardless of experience.
For more information please contact NDAS secretary Julie Buckingham at [email protected] or call
Mark Buckingham on 01271 345374.