APRIL 2018 1. SKY CHARTS - The Hermanus Astronomy Centre
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
APRIL 2018
1. SKY CHARTS
E V E N I N G S K Y M I D AP R I L a t 2 1 h 0 0 ( N O R T H D O W N )
E V E N I N G S K Y M I D AP R I L a t 2 1 h 0 0 ( S O U T H D O W N )
1
PLEASE NOTE: All events predicted below are as observed from Hermanus, Western Cape, South
Africa
2. THE SOLAR SYSTEM
PLANET VISIBILITY
Mercury Initially too close to the sun but becoming visible in the east before sunrise
Venus the “Evening Star” just after sunset
Mars Morning
Jupiter Observable throughout the night
Saturn Morning
Uranus Low in the west after sunset becoming visible low in the east before sunrise
Neptune Morning
Pluto Morning
Sun & Planets APRIL 2018 1st 30th
Sun Rises: 06h55 07h18
Constellation Pisces to Aries Transits: 12h47 12h40
Length of day 11h43 to 10h46
Sets: 18h38 18h03
Mercury Φ 11” to 8” Rises: 07h04 05h07
phase 0% to 45% Transits: 12h45 11h03
Constellation Pisces to Cetus
Magnitude +5.3 to +0.4 Sets: 18h26 18h58
Venus Φ 11” Rises: 08h31 09h31
phase 94% to 89% Transits: 14h02 14h30
Constellation Aries to Taurus
Sets: 19h33 19h28
Magnitude -3.9
Mars Φ 9” to 11” Rises: 23h28 22h41
phase 88% Transits: 06h41 05h51
Constellation Sagittarius
Sets: 13h52 13h00
Magnitude +0.3 to +0.4
Jupiter Φ 43” to 45” Rises: 20h33 18h29
Constellation Libra Transits: 03h27 01h21
Magnitude -2.4 to -2.5
Sets: 10h17 08h09
Saturn Φ 17” Rises: 23h33 21h40
Constellation Sagittarius Transits: 06h44 04h50
Magnitude +0.5 to +0.4
Sets: 13h50 11h57
Uranus 3” Rises: 08h13 06h27
Constellation Pisces to Aries Transits: 13h47 11h59
Magnitude +5.9
Sets: 19h21 17h32
Neptune Φ 2” Rises: 04h50 03h00
Constellation Aquarius Transits: 11h11 09h20
Magnitude +8.0 to +7.9
Sets: 17h31 15h39
Pluto Rises: 00h33 22h35
Constellation Sagittarius Transits: 07h36 05h43
Magnitude + 14.3 to 14.2
Sets: 14h40 12h47
Notes to the table above on the following page ...
2
Phase: In a telescope, the inner planets (Mercury, Venus and Mars) appear to us in phases, depending on
the angle of the Sun’s illumination, as does the Moon. The angular diameter (φ) is given in arc seconds (“).
This is the apparent size of the object as we see it from Earth. To illustrate this point, consider the average
binoculars through which we see about 7º of sky. Therefore, for example, Mars at 19” on 1st May would cover
approximately 1/1300th of the field of view.
Magnitude: we are accustomed to hearing stars described in terms of ‘magnitude’, for example Antares (in
Scorpius) at +1.05 and the planet Jupiter, at magnitude -1.9. The latter is considerably brighter than Antares
as the scale is ‘inverse’; the brighter the object, the lower the number. A ‘good’ human eye on a clear night
can see down to a magnitude of about +6.
Transit: When an object crosses the local meridian it is said to ‘transit’. The local meridian is an imaginary
line from the horizon directly north passing overhead to the horizon directly south.
THE MOON
Lunar Highlight (information from the 2018 Sky Guide Africa
South):
Theophilus
Type: Deep crater, with an imposing central mountain
(having four summits) rising 1 400m above the crater
floor.
Size: 104 km.
Notes: With Cyrillis and Catharina, they form a lovely
trio of linked craters. Named after the 4th century
Egyptian, Theophilus, 23rd pope of Alexandria.
Best seen: four days after March Full Moon (4th April)
and five days after New Moon (21st April).
Age: greater than 3.8 billion years
Location: north-western border of Mare Nectaris.
Also please see note 3 following “Highlights from the Sky
Guide” on page 4.
Eclipses (visible from Southern Africa):
No eclipses, solar or lunar, are predicted for this month.
METEOR SHOWERS
Date & Time Observing
Name Duration Radiant ZHR velocity
of Max Prospect
δ Pavonids 6th April 11th March About 7º south of
5 59 Unfavourable
02h00 to 04h30 to 16th April α Pav
April Lyrids 22nd April 16th to 25th About 3º SW of
15 49 Favourable
02h00 to 05h00 April Vega (α Lyr)
3
3. HIGHLIGHTS FROM THE SKY GUIDE
(viewed from Hermanus)
Date Time Item
1 Mercury at inferior conjunction
2 Mars near Saturn
3 Moon near Jupiter
7 Moon near Mars and Saturn
Moon furthest south (-20.3º)
8 09h18 Last quarter Moon
07h33 Moon at apogee (404 144 km)
Moon near Pluto
12 1
Yuri’s Night
13 Moon near Neptune
14 Moon near a (stationary!) Mercury
16 03h57 New Moon
Moon near Uranus
17 Saturn at aphelion
18 Moon near Venus
Saturn stationary
Uranus at conjunction
19 06h00 Moon 1.25º north of Aldebaran
20 16h46 Moon at perigee (368 712 km)
21 Moon furthest north (+20.4º)
22 23h46 Moon first quarter
2
Earth day
22h42 3
Lunar-X visible
April Lyrid meteor show at maximum (see METEOR SHOWERS p. 3)
23 Mercury at aphelion
Pluto stationary
24 Mars near Pluto
25 00h40 Moon 1.6º north of Regulus
29 Mercury at greatest eastern elongation (27º)
Comet 169P/NEAT at perihelion (period 4.2 years)
30 02h58 Full Moon
1
Yuri's Night is an international celebration held every April 12 to commemorate milestones in space
exploration. Yuri's Night is named for the first human to launch into space. Yuri Gagarin flew the Vostok 1
spaceship on April 12, 1961.
2
Earth Day is an annual event celebrated on April 22. Worldwide, various events are held to demonstrate
support for environmental protection. First celebrated in 1970, Earth Day events in more than 193 countries
are now coordinated globally by the Earth Day Network.
3
From Wikipedia - The Lunar X (also known as the Werner X) is a clair-obscur effect in which light and
shadow creates the appearance of a letter 'X' on the rim of the Blanchinus, La Caille and Purbach craters.
The X is visible only for a few hours before the first quarter, slightly below the lunar terminator. Near to the
X, the Lunar V is also visible, formed by Ukert crater and several other small craters.
4
4. STARGAZING
SUGGESTED OBSERVATION DAYS FOR APRIL:
Unless specifically targeting the moon, I suggest the most convenient dates to plan evening
stargazing in April are 6th (moonrise 22h52) to 20th (moonset 22h04).
The next club stargazing evening is
yet to be planned. Please see the
calendar of events on our website for
updates:
http://www.hermanusastronomy.co.za
NO ‘SCOPE REQUIRED (or Getting to Know The Constellations )
I offer here some more tips for the less experienced enthusiast on getting to know the constellations
and identifying some of the lesser-known features of the night sky. Toes north-west this time!
The chart to right depicts the north-western
evening sky in mid-April with particular
reference to Orion.
We are all familiar with the dominant
summer constellation Orion. Well-known
are the right shoulder Betelgeuse (α Ori,
+0.45 mag., 500 LY distant) and left leg
Rigel (β Ori, +0.15 mag., 860 LY away). In
the middle of the sword lies the famous
and glorious nebula M42.
Lesser-known are the left shoulder
Bellatrix and right leg Saiph. The head (a
small star suggesting he wasn’t too bright?)
is Meissa.
The three stars of the belt are:
Alnitak (double star, +1.85, 817 LY)
Alnilam (variable, +1.65, 1 977 LY)
Mintaka (double, +2.4, 917 LY)
Surrounding the hunter are his dogs, Canis
Major and Minor separated by Monoceros. Then Gemini with Alhena (left foot of Pollux) and Tejat
(left foot of Castor). Then Taurus the bull, Eridanus the river and Lepus the hare.
See Ian Ridpath’s “Star Tales” (below). Makes some interesting reading. Note his comments on the
connection of Orion with Heracles (Hercules).
5From Ian Ridpath’s “Star Tales”
Genitive: Orionis
Abbreviation: Ori
Size ranking: 26th
Origin: One of the 48 Greek constellations listed by Ptolemy in
the Almagest
Greek name: Ὠρίων
Orion (Ὠρίων in Greek) is the most splendid of constellations, befitting a character who was in legend
the tallest and most handsome of men. His right shoulder and left foot are marked by the brilliant stars
Betelgeuse and Rigel, with a distinctive line of three stars forming his belt. ‘No other constellation
more accurately represents the figure of a man’, says Germanicus Caesar.
Manilius called it ‘golden Orion’ and ‘the mightiest of constellations’, and exaggerated its brilliance by
saying that, when Orion rises, ‘night feigns the brightness of day and folds its dusky wings’. Manilius
described Orion as ‘stretching his arms over a vast expanse of sky and rising to the stars with no less
huge a stride’. In fact, Orion is not an exceptionally large constellation, ranking only 26th in size
(smaller, for instance, than Perseus according to the modern constellation boundaries), but the
brilliance of its stars gives it the illusion of being much larger.
Orion is also one of the most ancient constellations, being among the few star groups known to the
earliest Greek writers such as Homer and Hesiod. Even in the space age, Orion remains one of the
few star patterns that non-astronomers can recognize.
In the sky, Orion is depicted facing the snorting charge of neighbouring Taurus the bull, yet the myth
of Orion makes no reference to such a combat. However, the constellation originated with the
Sumerians, who saw in it their great hero Gilgamesh fighting the Bull of Heaven. The Sumerian name
for Orion was URU AN-NA, meaning light of heaven. Taurus was GUD AN-NA, bull of heaven.
Gilgamesh was the Sumerian equivalent of Heracles, which brings us to another puzzle. Being the
greatest hero of Greek mythology, Heracles deserves a magnificent constellation such as this one,
but in fact is consigned to a much more obscure area of sky. So is Orion really Heracles in another
guise? It might seem so, for one of the labours of Heracles was to catch the Cretan bull, which would
fit the Orion–Taurus conflict in the sky. Ptolemy described him with club and lion’s pelt, both familiar
attributes of Heracles, and he is shown this way on old star maps. Despite these parallels, no
mythologist hints at a connection between this constellation and Heracles.
Tales of Orion
According to myth, Orion was the son of Poseidon the sea god and Euryale, daughter of King Minos
of Crete. Poseidon gave Orion the power to walk on water. Homer in the Odyssey describes Orion as
a giant hunter, armed with an unbreakable club of solid bronze. In the sky, the hunter’s dogs (the
constellations Canis Major and Canis Minor) follow at his heels, in pursuit of the hare (the
constellation Lepus).
On the island of Chios, Orion wooed Merope, daughter of King Oenopion, apparently without much
success, for one night while fortified with wine he tried to ravish her. In punishment, Oenopion put out
Orion’s eyes and banished him from the island. Orion headed north to the island of Lemnos where
Hephaestus had his forge. Hephaestus took pity on the blind Orion and offered one of his assistants,
Cedalion, to act as his eyes. Hoisting the youth on his shoulders, Orion headed east towards the
sunrise, which an oracle had told him would restore his sight. As the Sun’s healing rays fell on his
sightless eyes at dawn, Orion’s vision was miraculously restored.
6Orion is linked in a stellar myth with the Pleiades star cluster in Taurus. The Pleiades were seven
sisters, daughters of Atlas and Pleione. As the story is usually told, Orion fell in love with the Pleiades
and pursued them with amorous intent. But according to Hyginus, it was actually their mother Pleione
he was after. Zeus snatched the group up and placed them among the stars, where Orion still
pursues them across the sky each night.
© Ian Ridpath. All rights reserved
5. DEEP SKY HIGHLIGHT (from the Sky Guide Africa South 2018)
The Ghost of Jupiter (NGC 3242)
This large planetary nebula in Hydra was discovered in 1785 from England by William Herschel. He
called it “a beautiful, very brilliant globe of light ... the colour of Jupiter.”
With a diameter of about 40” it is similar in apparent size to the giant planet. John Herschel, while at
the Cape some 50 years later, described it has having “a decided pale blue colour.” Other observers
have reported it as appearing green! Through high-power binoculars or a small telescope NGC 3242
appears as a bloated star and can be easily picked out from the surrounding star field
A fair bit of careful star-hopping is required to pin down this “ghostly” apparition but seeing conditions
need to be very good! My suggestion:
Refering to the north down sky chart on page 1, using Sirius, Procyon and Regulus as
guide stars, locate Alphard (α Hya, magnitude +1.95).
From Alphard, move east-south-east 17º to µ Hya (mag. +3.8). Thence south-south-east
1.8º to The Ghost of Jupiter (mag. +9.0).
[by the way, a fist at arm’s length is about 15º arc across the sky]
7Please keep in touch...
Don’t forget to have a look at our excellent website, edited by Derek Duckitt.
http://www.hermanusastronomy.co.za/
Also...
ASSA website http://assa.saao.ac.za
ASSA Deep-Sky Section
Whatsapp chat group: [ 074 100 7237 ]
Official Big 5 of the African Sky web page
Official Big 5 Facebook group
ASSA Deep-Sky Section mailing list
Contact ASSA
Get in touch with officers of the Society - we're real people with a passion for
astronomy, so contact us and let's talk!
You can find us on Facebook, Twitter, the ASSA Info mailing list and the ASSA
Discussion mailing list.
Grateful thanks to the following, without whom this publication just would not be the
same:
ASSA
Sky Guide Africa South 2018
Stellarium
Ian Ridpath
Mick Fynn
Edited by Peter Harvey
e-mail: petermh@hermanus.co.za
Tel: 081 212 9481
8You can also read
