Explore the final month of Summer skies with:
The Australian
February 2017
Night Sky
By Wayne Roberts
The times, circumstances and star charts to follow are
configured for the Melbourne Observatory in line with other
publications of the Astronomical Society of Victoria (ASV), for
which these viewing notes are principally compiled: Longitude 144°
58' 23.8" E, Latitude 37° 49' 54.1" S [" denotes arc-second =
1/60th of an arc-minute (symbol ') or 1/3600th of a degree (symbol
°)].
The notes contain many references to rise and set times, all
configured for the co-ordinates given above; the following
procedure will facilitate determining the correct times for other
locations.
The adjustment required for longitude is straight forward – for
each degree east (of Melbourne Observatory) subtract four minutes;
conversely, add four minutes for each degree west.
Adjustment for a difference in latitude is dependent on the
declination of the object concerned – how far north or south of the
celestial equator it lies. The table below, from the yearbook of
the ASV, gives the required adjustment (in minutes) for any object
at a declination of between +30° and -30° (by convention, north is
positive and south negative) when viewing from any location between
latitudes -28° and -44°; interpolate for intermediate
latitudes/declinations. Note: add these values to times of rising
and subtract from setting times.
Whereas the declinations of stars and other deep sky objects are
(very nearly) constant, those of the Sun, Moon and planets change
considerably over time; a suitable resource for determining
declinations is therefore required to obtain the adjustment for
difference in latitude as described above.
While the reader may have ready access to such resources, I
offer the following suggestions for those who do not: the ASV
yearbook, free to members and available at reasonable cost to
others (Sun and planets only); the yearly publication Astronomy
Australia 2016 (Sun, planets and Moon); and various astronomical
software packages (typically catering for all celestial objects).
Other sources may be available on-line.
Note that some resources give co-ordinates for epoch 2000.0 –
denoted as J2000 – whereas others may quote real time figures,
denoted JNow. The discrepancy between the two systems will not be
significant for the purpose at hand.
Now a few notes defining terms which may be encountered in the
text to follow:
For those not familiar with the terminology used to describe
phases of the Moon, note that it is referred to as waning (hard
‘a’) when the phase is decreasing daily and waxing (soft ‘a’) when
increasing, and as a crescent when less than half lit and gibbous
when more than half. Thus you may find it referred to in the text
as, for example, a waxing crescent Moon.
Appearances of the abbreviation “au” in the text denote
“astronomical unit”, the average Earth-Sun distance, currently
defined as 149,597,870.7 km.
The abbreviation ZHR, where it appears in relation to meteor
showers, refers to the zenith hourly rate, the number of meteors
per hour which can be expected to be seen under ideal conditions,
with the radiant (the point from which the meteors appear to
emanate) at the zenith and a clear dark sky.
The ecliptic, shown as a green line on many of the star charts
to follow, is the path followed by the Sun and (very nearly) the
planets as they arc across our sky daily.
As the outer planets, those that orbit farther from the Sun than
does Earth, move along their orbital paths, they move from west to
east in our sky relative to the stars; this is referred to as
direct or prograde motion. Around the time of opposition however,
when we overtake them on our inner, faster orbital path, they
change direction, moving east to west relative to the stars.
Referred to as retrograde motion, this phenomenon is caused by the
same effect as that seen when a car which is overtaken seems
momentarily to move backwards in relation to a line of trees in the
background.
On any given night, a planet is said to be transiting (or
culminating, there is a subtle difference between the two) when it
reaches its highest point above the horizon in its passage across
our sky that evening – do not confuse this use of the term
‘transit’ with the same word often used in these viewing notes to
describe the passage of the moons of Jupiter across the face of
their parent.
Regarding the inner planets, Mercury and Venus, which never
stray very far from the Sun in our sky, greatest eastern elongation
refers to their maximum angular distance east of the Sun when they
are visible in our evening skies; similarly, greatest western
elongation refers to their maximum angular separation west of the
Sun when visible in our morning skies. They are said to be in
inferior conjunction when passing between Earth and the Sun on
their inner orbital tracks, and in superior conjunction when
rounding the far side of the Sun from our perspective.
As a handy (excuse the pun) guide to estimating the angular
separation of two objects, one finger held at arm’s length
typically spans a little over 1°, a closed fist 10° and an open
hand, thumb tip to tip of little finger, 20°.
Note finally that perihelion or aphelion of Mercury, Venus and
other planets refer to the physical separation in space of the
planet in question and the Sun, and are unrelated to their angular
separation in our sky.
What’s in the sky this month; February 2017:
4th First Quarter Moon.
6th Aldebaran (Alpha [α] Tauri, magnitude 0.8), occulted by Moon
(not from Australia).
7th Moon at perigee (closest to Earth, 368,816 km);
Jupiter stationary, begins retrograde motion.
8th Mercury at aphelion (farthest from Sun, 0.4667 au / 69.82
million km);
Alpha Centaurid meteor shower peaks.
11th Full Moon;
Penumbral eclipse of Moon (not from Australia).
12th Regulus (Alpha Leonis, magnitude 1.4) occulted by Moon (see
below).
17th Jupiter at aphelion, 5.4565 au / 816.3 million km;
Venus greatest brilliancy (aka greatest illuminated extent),
magnitude -4.63.
19th Last quarter Moon;
Moon at apogee (farthest from Earth, 404,376 km).
21st Venus at perihelion (closest to Sun, 0.7185 au / 107.5
million km).
24th Delta [δ] Leonid meteor shower peaks.
27th New Moon;
Annular eclipse of the Sun (not from Australia).
N.B.: When reading the following, refer back to the explanatory
notes at the beginning of this article for information on
terminology, angular separation approximations and adjustment of
latitude & longitude. Note also that the occurrences of
aphelion and perihelion shown above relate to the physical
separation of the Sun and the relevant planet, and are unrelated to
their angular separation in our sky.
The occultation of Aldebaran by the Moon on the 6th is only
visible from Central America, the Caribbean and Southern Europe.
For us, it’s a non-event as only a close conjunction results, with
the star just over ¼° from the Moon’s limb, and closest approach,
at 8:37 am, taking place almost 70° below our northern horizon. The
circumstances of the Regulus occultation on the 12th are much
kinder – the star is occulted, high in our northern sky, from 12:28
am until 1:50 am.
Neither of the eclipses mentioned above – a penumbral[a
penumbral eclipse of the Moon occurs when it passes through only
the diffuse outer portion of our planet’s shadow, with the Earth
never fully covering the Sun’s disk from anywhere on the lunar
surface] eclipse of the Moon on the 11th and an annular[an annular
solar eclipse occurs when the Moon is too far from the Earth for
the shadow thrown by the Moon to reach Earth’s surface, whereby we
see a ring of fire surrounding Luna] eclipse of the Sun on the 27th
– are visible in Australian skies. The lunar eclipse can be seen
from locations in South America, Europe, Africa and western Asia;
from our location, having set at 6:38 am, the Moon is below the
horizon by 36° when the eclipse begins and 65° when it ends. The
solar eclipse of the 27th can be viewed from Chile, Argentina, a
path across the Atlantic Ocean and parts of Africa. The Moon’s
shadow doesn’t touch Australia; maximum eclipse occurs over the
Atlantic Ocean at 1:58 am our time.
The Alpha Centaurid meteor shower peaks on the 8th, at the
unfortunate time of 11:30 am. This shower, which under ideal
conditions usually results in around six meteors per hour streaking
across our skies, is very heavily impacted by the 85% illuminated
waxing gibbous Moon, which doesn’t set until 3:30 am, leaving just
a 1½ hour dark sky window before morning twilight commences at 5:02
am. As the radiant[the point from which the meteors appear to
emanate] is circumpolar, never setting, and will be at an altitude
of 57° at the beginning of the window of opportunity, improving to
66° at the end, some worthwhile activity may be forthcoming during
this brief period. Accordingly, the following chart shows the
location of the radiant as the Moon sets; also marked is the
radiant of the almost incidental Delta Leonids, which peak on the
24th, with an unspectacular ZHR of 1.
THE PLANETS
Mercury
As February begins, Mercury is in the declining half of a
morning apparition in which it just barely manages to break free of
twilight, between January 20th and February 5th (my apologies for
indicating last month that it wouldn’t quite do so, it actually
beats twilight to the punch by a maximum of 6½ minutes on Jan
27th). It rises (on Feb 1st) at 4:47 am, and is at an altitude of
19° when the Sun breaches the eastern horizon at 6:34 am. The
innermost planet’s disk spans 5.6" [" denotes arc-second = 1/60th
of an arc-minute (symbol ') or 1/3600th of a degree (symbol °)] and
is 81% illuminated; at magnitude -0.2, the planet will be clearly
visible to the naked eye before twilight washes it out.
As the month progresses, Mercury descends towards the eastern
horizon at an ever increasing pace. By our viewing night, which
we’ll nominate as Saturday 25th (New Moon falling on the following
Monday), it’s effectively run its race, rising less than ¾ hour
before the Sun (6:16 am vs 7:00 am) and attaining an altitude of
just 8° come sunrise. While its disk, 97% lit and spanning 4.9" as
it increases its distance from Earth, shines more brightly at
magnitude -0.9, it will be swamped by twilight (note that Mercury,
alone amongst all the planets, is brightest when far from Earth and
dimmest when near us, due to…what? Feel free to contact me via the
address at the end of these viewing notes if you’re unable to
resolve this).
Come the end of the month, the data of the 25th read: a little
over ½ hour, 6:31 am, 7:03 am, 6°, 98%, 4.9" and mag -1.1.
Because it will be an unviable target later in the month, the
following chart shows where it sits, 8° clear of the horizon, at
5:30 am on the 1st; the constellation figures of Scorpius and
Sagittarius are plotted, as are Saturn and Pluto for later
reference.
Beginning February in the constellation of Sagittarius, fleet
footed Mercury moves into Capricornus on the 8th and Aquarius on
the 25th.
Venus
February is an exciting time for those of us who view Venus
through a telescope; while it loses a little altitude nightly, its
disk grows in size and shrinks in phase quite dramatically as it
blazes fiercely in the western sky.
It begins the month 21° clear of the horizon at sunset, 8:33 pm,
and doesn’t itself set until 10:23 pm; its disk, spanning an
already impressive 31", presents as a pleasing 39% lit crescent,
shining as only it can (none bar the Sun and Moon, and occasional
Iridium flash, are brighter; it would easily outshine the Moon were
it of comparable size in our sky) at a punishing magnitude
-4.56.
Venus reaches its greatest brilliancy – known as greatest
illuminated extent – at a staggering magnitude -4.63 on the 17th.
Come our viewing night of the 25th, it is setting considerably
earlier, at 9:05 pm, having been at an altitude of 11° when the Sun
set just under one hour earlier, at 8:06 pm. The planet’s earlier
setting time is, however, more than compensated for by the evolving
circumstances of its disk – it then spans a whopping 44", as a
beautiful 20% illuminated crescent, still shining brilliantly at
magnitude -4.61. By month’s end, the above read 8:52 pm, 9°, 8:01
pm, 46" and mag -4.59.
Be sure to follow the scintillating orb throughout February and
the first week or so of March as its span increases to in excess of
50" and the crescent phase becomes ever slimmer – it really is a
fantastic sight, just be sure to wait until the Sun is below the
horizon to protect your eyesight.
Venus remains within Pisces throughout February.
Mars
While the Red Planet will adorn our western skies for some
months to come, not reaching conjunction with the Sun until the
last week of July, its days of doing so in a fully dark sky are
effectively over even at the start of February. No other planet
lingers in the west so tenaciously however; Mars does so for two
reasons: it is our relatively very close outer neighbour, whereby
we, on our inner and faster orbital track, pull away from it only
slowly, and also because, for this apparition, Mars’ retreat
towards the western horizon coincides with the time of the year
when the Sun is setting earlier each night, causing the sky to
progressively darken earlier while Mars is still some way above the
horizon.
On Feb 1st, Mars is only 25° above the WNW horizon at sunset
(8:33 pm), less than 5° when the sky fully darkens at 10:14 pm, and
sets at 10:45 pm; its 92% illuminated disk spans just 5.1" (too
small to see detail, and probably also to detect the less than full
phase) and shines at magnitude 1.11. Wait until the sky darkens
sufficiently to see Venus, Mars and the 21% illuminated waxing
crescent Moon forming a straight line spanning just 9°.
On the 25th, our designated viewing night, the situation has
deteriorated somewhat – the planet sets at 9:53 pm after having
been at an altitude of 20° when the Sun set at 8:06 pm, and less
than 2½° when twilight faded at 9:38 pm; its 94% illuminated disk
is both smaller, 4.7", and fainter, magnitude 1.27. The following
chart, configured for 8:51 pm, ¾ hour after sunset and well within
twilight, gives an indication of just how low Mars sits before the
sky darkens sufficiently to invite viewing. Note also the position
of Uranus, the closeness of which to Mars will be further discussed
in the notes on the gas giant.
On the 28th, Mars sets at 9:46 pm, less than ¼ hour after the
cessation of twilight (9:33 pm). Still at a phase of 94%, its 4.6"
disk has further dimmed to magnitude 1.29.
Beginning the month swimming with the fishes in Pisces, Mars
just barely cuts through a corner of Cetus (the whale or sea
monster) on the 7th, before resuming its accompaniment of the small
fry for the rest of the month.
Jupiter
The King of the Planets is now rising before midnight, and so
becoming a fixture in the evening, as well as the morning, sky. At
the start of the month it rises less than ½ hour before midnight,
11:33 pm, and transits at 5:59 am, a little over ½ hour before
sunrise (on the 2nd), its disk spanning 39.0" and dominating the
sky at magnitude -2.15. Jupiter is stationary on the 7th, as it
begins four months of retrograde motion, moving east to west
relative to the stars. Rise and transit times have improved
markedly by the 25th, to 9:58 pm and 4:23 am (on the 26th, almost
an hour before twilight begins at 5:28 am) respectively; its disk
is also bigger and brighter at 41.8" and mag -2.30.
Due to Jupiter’s brilliance, no help is needed to pinpoint it –
just look for the brightest ‘star’ in the sky (it rises in the
east, don’t mistake it for brilliant Venus in the west). The
following magnified view is configured for 12:30 am on the morning
of the 26th, with Jupiter 29° high in the ENE. The time has been
chosen to catch Europa and Callisto just 24" apart, as the former
reaches its maximum eastern elongation from Jupiter, thereafter
moving west from our perspective, back towards Jupiter, while
Callisto, with a wider orbit, continues to move away from its
parent. If you plan to view late into the morning, you’ll see the
gap between the two moons widen noticeably. Ganymede and Io are to
Jupiter’s west, with the former moving away from its parent while
Io is about to begin moving back to the east from our perspective.
Even the brightest of the stars shown shines faintly at magnitude
13.7 (probably remaining unseen through the eyepiece in other than
large amateur ‘scopes) and so none can be mistaken for the 5th and
6th magnitude moons.
As February concludes, Jupiter’s rising time stands at 9:46 pm,
its disk spanning 42.1" and shining at magnitude -2.32; the giant
planet will glide among the stars of Virgo until November.
Saturn
Saturn and its marvellous ring system have now become a fixture
in our morning skies, affording fascinating views to those who view
through into the wee hours.
Before delving into its circumstances this month, I’ll briefly
address the problem raised last month in relation to Saturn
increasing in brightness in the lead up to conjunction and
decreasing afterwards, contrary to the minimum at conjunction that
conventional wisdom would dictate.
Determined to come to grips with this counter-intuitive
behaviour, I examined the data relating to all the outer planets,
Mars through Pluto, for the period 2014-2017, and found what I can
only describe as a dog’s breakfast (with apologies to man’s best
friend); in a nutshell, here are my findings:
MARS – dims up until conjunction, levels out around conjunction,
with a propensity to brighten marginally (by a factor of magnitude
0.01) for a few days, then resumes dimming for 2-3 months
afterwards.
JUPITER – at its dimmest anywhere between a week before
conjunction and a couple of weeks after.
SATURN – brightens for 50-60 days in lead up to conjunction,
then dims for a similar period after conjunction.
URANUS and NEPTUNE – dimmest at conjunction.
PLUTO – brightens for 35-55 days in lead up to conjunction, dims
for similar period after conjunction.
As the above shows, the planets exhibit a range of behaviours in
the period before and after conjunction – I remain unable to
reconcile this, other than to say that Saturn & Pluto appear to
behave in a like manner, as do Uranus & Neptune, while both
Mars and Jupiter follow their own unique regimes. All I can do at
this stage is flag the particular trends as each plays out, and
commit to reporting back in future editions of these viewing notes
if clarification ensues. If any readers can help to resolve this,
please do so.
At the beginning of February, Saturn is rising at 2:56 am, and
is 21° (vertically) clear of the horizon when morning twilight
commences at 4:53 am. Its disk spans 15.6", the rings 35.3" at an
angle of 26.7°; disk and rings together shine at magnitude
0.54.
When viewing on the 25th, you’ll find the planet rising in the
east at 1:29 am and attaining a much more pleasing altitude of 45°
at the commencement of twilight (5:27 am). The span of the disk and
rings will have improved to 16.1" and 36.5" respectively, with the
latter inclined at 26.6°; at the end of the month, the above
figures stand at 1:18 am, 48°, 16.1", 36.6" and 26.5°.
Refer to the meteor and/or Mercury charts to determine where
Saturn sits in the sky. While those charts are configured for the
8th and 1st respectively, Saturn will be at very nearly the same
position relative to the stars on our viewing night. At 4:00 am on
the 25th it will actually be within about 1° of the altitude
depicted in the Mercury map, and this is the time – just under ½
hour before twilight starts to paint the sky – for which the
magnified views, on the following page, of Saturn and its seven
brightest satellites are configured. Saturn is at an altitude of
28°, very near the 30° this column recommends in order to totally
avoid the murk found near the horizon.
While Saturn is currently traversing the rich star fields of
Sagittarius, the brightest stars in the field of view afforded by
the chart are 15th magnitude, far dimmer than, and not able to be
mistaken for, the moons, and have consequently been omitted for the
sake of clarity.
The magnitude rating of the brightness of the moons is as
follows: Titan 8.8, Enceladus 12.2, Dione 10.9, Mimas 13.4, Tethys
10.7 and Rhea 10.2. Titan will be visible through any telescope,
even finder ‘scopes and most binoculars, although faint; Rhea,
Tethys and Dione (as well as Iapetus, see below) should show up in
a four to six incher, while Enceladus may call for an eight to ten
incher and tiny Mimas a twelve incher.
The following chart includes Iapetus, which has a much wider
orbit than the others:
Enigmatic Iapetus has one hemisphere much brighter than the
other, whereby its brightness in our sky is considerably more
affected by its 79 day orbit around Saturn – being brightest when
to the west of the planet and dimmest when to the east – than the
Earth-Saturn separation, which cycles over roughly 12 months.
Accordingly, whereas the brightness of the other moons as given
above is taken from Starry Night software, which unfortunately does
not account for the moon’s orbital position, that for Iapetus, mag
10.5, is my own (confident) estimate.
This latter chart retains the star field as an aid in
identifying the outlying moon. The brightest stars between Saturn
and Iapetus are 14th magnitude, and the closest stars to the moon
15th-17th magnitude, as are stars beyond Iapetus for well past the
extremes of the chart, the one exception being the unlabelled
relatively bright star at top left (which, even so, is only
magnitude 14.4, far dimmer than the moon and unseen through all but
large ‘scopes). Identification should therefore not be too
difficult – just look in the direction indicated, at the
appropriate distance from Saturn, for the lone relatively bright
‘star’. Iapetus is only 7' – just a little over 0.1° – from Saturn
and thus both will be in the same field of view even at high
magnification. Note that whereas the software normally does a good
job of depicting brighter objects as larger, it fails in this case
– all the moons except Mimas are plotted at the same size, even
though there is considerable variation in brightness. Mimas itself
is too small as compared to the others, and the stars, being as
faint as they are, are shown as being too big in relation to the
moons.
Saturn will transition from Ophiuchus to Sagittarius on the
24th; retrograde motion will actually return it to Ophiuchus in the
second half of May, before it again crosses the border on November
19th after prograde motion (west to east relative to the stars)
resumes.
Uranus
Uranus will be in conjunction with the Sun in mid-April, and as
such it is even now becoming too low in the west for decent viewing
once darkness has descended. Its brightness declines ever so
slightly from 5.85 to 5.88 over the course of February, the span of
the disk from 3.5" to 3.4".
It is still accessible on the 1st, setting almost three hours
after the Sun (11:30 pm vs 8:33 pm); however it only sits at an
altitude of 14° once twilight fully fades at 10:14 pm (and with a
21% waxing crescent Moon lingering). By the 25th, it’s becoming
quite marginal – setting time is 9:58 pm as compared to 8:06 pm for
Sol, and the planet sits just 3½° clear of the horizon at 9:38 pm,
the cessation of twilight.
The above chart is configured for the 25th at 9:06 pm, one hour
after sundown, with Uranus and Mars at an altitude of 9½° and 8½°
respectively, some 1¼° (around the width of a finger at arm’s
length) separating them. As indicated earlier, Uranus has a close
conjunction with Mars this month, closest approach of a little over
½° occurring on the night of the 27th; green crosses show the
position of the two on that night. The brightest stars in the
vicinity are labelled with their magnitudes to emphasise the fact
that magnitude 5.88 Uranus is easily identified through a finder
‘scope by reference to Mars, barring too much atmospheric
interference at the low altitude. Once you’ve spotted it through
your finder, switch to the main eyepiece at a magnification of 150x
or more to resolve its wee disk.
At the end of February, Uranus sets at 9:46 pm, being just 2°
high when twilight wraps up at 9:33 pm; the gas giant is in Pisces
until 2018/19.
Neptune
Neptune is even further advanced in its current apparition than
its inner brethren, viewing being affected, or even essentially
disqualified, by twilight all month; the planet reaches conjunction
on the 2nd of next month.
Even as February gets underway it sets within twilight, a little
over 1¼ hours after the Sun sets (9:52 pm/8:33 pm), when it’s at an
altitude of just 15°. The situation is truly hopeless on our
viewing night of the 25th, with the planet setting at 8:20 pm, less
than ¼ hour after the 8:06 pm sunset, while at month’s end, the
above data read 8:08 pm, 7 minutes and 8:01 pm.
Neptune’s brightness declines from magnitude 7.95 to 7.96
throughout February, its disk spanning 2.2" all month. Views of the
outermost planet proper (although Pluto is held in planetary status
by this column) are best held over until it reappears in the
morning sky and gains a little altitude.
Neptune, which chugs along at a gentle pace amongst the stars
due to its wide and slow orbital track, will remain within the
constellation of Aquarius until 2022/23.
Pluto
Pluto is a little better situated than Neptune, albeit on the
other side of conjunction, having reached that point early last
month. The Mercury chart provides an overview of where Pluto sits
at 5:30 am on the morning of the 1st, and at the resolution
provided by the chart it won’t move noticeably throughout February
thanks to its sluggish movement relative to the stars, slower even
than Neptune.
On the first of the month, it rises at 4:39 am, almost two hours
before the Sun breaches the eastern horizon at 6:36 am, but less
than ¼ hour before twilight commences (4:53 am). By the 25th it has
gained altitude, rising at 3:08 am (sunrise 7:00 am), but is still
just 26° high as the Sun’s light begins to filter across the sky at
5:27 am; three days later at the end of the month, these figures
read 2:57 am, 7:03 am, 29° and 5:31 am.
The span of this tiny frozen orb increases incrementally over
the course of the month from 0.093" to 0.094" (far too small to be
resolved by amateur instruments), while its visual magnitude
actually drops (as related in the earlier discourse on counter
intuitive planetary brightness around conjunction) from magnitude
14.29 to 14.30.
As Pluto is difficult to locate at the best of times, these
viewing notes only give detailed directions for doing so in the
months around and after opposition, when it can be found high in
the sky at a convenient hour. For now, here’s a magnified view
showing its approximate position at 5:00 am on the 25th, near the
trio of naked eye stars – Xi2 [ξ2] Sagittarii, mag 3.5, Omicron [ο]
Sag, mag 3.75, and Albaldah (Pi [π] Sag), mag 2.9 – below the
handle of the teapot asterism.
Pluto will remain in Sagittarius until 2023/24 (the first three
days only of the latter year).
OUR MONTHLY FEATURE
Asteroid 4 Vesta, the ‘4’ denoting that it was the 4th asteroid
to be found, after Ceres, Pallas and Juno – the first of which has
since been reclassified as a dwarf planet – was discovered on
29/3/1807 by the German astronomer Heinrich Olbers; in doing so he
became the first person to discover two asteroids, having
previously bagged Pallas (or 2 Pallas).
Named after the Roman goddess of home and hearth, Vesta is both
the second largest and second heaviest (after Ceres, which is
considerably more massive, Vesta only having 28% its mass) body
circulating in the asteroid belt between Mars and Jupiter, and
accounts for 9% of the total mass of bodies in the belt.
It is, on average, the brightest of all asteroids – brighter
even than Ceres, thanks in part to its slightly smaller orbit and
to its greater albedo. Albedo is the term used to describe the
percentage of light reflected by a body, 0 for a body which
reflects no light (and therefore appears totally black) and 1 for a
body which reflects all light incident upon it. Vesta’s albedo is
0.43; by comparison the albedos of a few other bodies are: Ceres
0.1, Earth 0.31, our Moon 0.11, Mars 0.25 and brilliant Venus,
which is covered with a thick layer of highly reflective white
cloud, 0.77. Vesta is, at its brightest, dimly visible to the naked
eye, and always visible in binoculars or a finder ‘scope, its
brightness varying from magnitude 5.1 to 8.5. At the time of
compilation of these viewing notes (mid-January) it shines at mag
6.1; come our viewing night of Feb 25th, this will have reduced
slightly to 6.3, reflecting (excuse the pun) the fact that it
reaches opposition, and therefore peak brightness, on Jan 18th.
This places it out of reach of the naked eye for all but those with
very keen eyesight under the darkest of skies, but it’s a prominent
object viewed at any degree of magnification.
Vesta orbits at a distance from the Sun which varies from 2.15
au - 2.57 au (322 million km - 384 million km) and can approach to
within 1.14 au (171 million km) of Earth. It has an orbital period
of 3.63 of our years, orbiting at an average velocity of 19.3
km/sec (as a comparison, Earth orbits at 29.8 km/sec) but a very
short ‘day’, rotating once on its axis in 5.34 hours. Lacking an
atmosphere to retain heat, Vesta’s surface temperature varies from
-3°C to -188°C.
Vesta is not quite massive enough for its gravity to have pulled
it into a spherical shape; it measures 569 km x 555 km x 453 km –
it is thought to have a nickel-iron core 214-226 km in diameter.
With a mass of 2.59*1017 tonne, objects dropped near its surface
accelerate downwards under the force of gravity at a leisurely 0.25
m/sec2 (as compared to 9.8m/sec2 on Earth and 1.6 m/sec2 on the
Moon).
NASA’s Dawn spacecraft orbited Vesta from 16-7-2011 until
departing for Ceres on 5-9-2012 (it was originally intended that
Dawn would depart from Ceres on July 1st last year en-route to
another asteroid, 145 Adeona, but the decision was taken to leave
it orbiting Ceres, where it remains today).
Vesta has two comparatively huge impact craters dominating its
southern hemisphere. The largest, dubbed Rheasilvia, is around 500
km wide, almost as wide as the asteroid itself, 19 km deep at its
lowest point, and contains a central peak 23 km high; the rim of
the crater formed by the impact reaches up to 31 km above the
floor. There is another crater not far behind in terms of size,
Veneneia, some 400 km across; Rheasilvia is younger and overlays
its smaller brother. Many relatively small bodies, which along with
Vesta are collectively known as the Vestian asteroids, were
probably blasted off the surface by the impact which created
Rheasilvia, as were the HED (howardite-eucrite-diogenite)
meteorites, which make up around 5% of all meteorites striking
Earth. Here’s an image, courtesy of NASA, taken by Dawn:
Vesta’s circumstances for the month of February, for the 1st,
25th and 28th respectively in each case, are:
Rise 7:41pm, 5:56 pm, 5:43 pm;
Transit 12:20 am (on the 2nd), 10:29 pm, 10:17 pm; transit
altitude 28°, 26°, 26°;
Twilight ends 10:14 pm, 9:38 pm, 9:33 pm; at altitude 21°, 25°,
25°;
Disk span and visual magnitude 0.42" & 6.13, 0.38" &
6.33, 0.38" & 6.36. Note that Vesta’s span is too small to
allow it to be resolved by amateur instruments.
All star charts courtesy of StarryNight®ProTM Version
7.5.5.1109/Simulation Curriculum Corp.
The wide field chart on the previous page shows where Vesta sits
in the sky on the 25th at time of transit (10:29 pm); below it is a
magnified view, in which all stars brighter than magnitude 8.0 are
labelled with their magnitudes. Relate the wide field chart to the
magnification using the naked eye stars Iota [ι] Gem, mag 3.75
& Upsilon [υ] Gem, mag 4.03, which are comfortably the
brightest stars in their immediate locale – they are clearly,
albeit faintly, visible on the wide field chart just beneath the
marker for Vesta, to the upper left of Pollux (Beta Geminorum, mag
1.15).
Having identified Iota & Upsilon Gem, train your finder
‘scope on the area above them, as indicated by Vesta’s position on
the chart. As the asteroid is the next brightest ‘star’ on the
chart (barring the stars of mag 5.0 and 5.75 near the bottom, with
‘6.34’ being essentially equal in brightness), identification
should not be difficult – the star of magnitude 7.31 (HIP36262), ½°
to Vesta’s lower left and the brightest star close to the asteroid,
may help to confirm its identity. If you’re still not sure, sketch
or image the brightest stars in the field and return the next night
to see which one has moved.
That’s all we have for this month folks, tune in next month when
The Australian Night Sky returns with another tour of the
heavens.
As usual, any comments, suggestions or questions are welcome,
and may be directed to [email protected]
Until next month:
HAPPY
HUNTING
CLEAR
SKIES
and
