Around the Sky in 28 Days – Day 23

Sorry about missing yesterday’s installment which was to focus on the Crab Nebula and the 1054 AD supernova that created it. Recently arrived newborns (and yes newborns is meant to be plural) and really make you forget about the time!

Tonight the Moon doesn’t get high in the sky until after midnight. With the Moon in Gemini, it is among the stars of the winter Milky Way. Many of these stars are bright enough to be seen even in the brightest of cities. Even entire constellations, such as Orion, may be seen.

Recent Discoveries – Sept 17-24, 2010

Catching up on the last week in comet and asteroid discoveries… A week ago new NEA announcements were coming left and right but the flood completely stopped a few days before Full Moon. Most surveys take a break for a few days around Full Moon since the bright sky is really not conducive for finding much of anything.

Last week saw one and probably two comet discoveries. C/2010 S1 (LINEAR) is a large perihelion (q = 4.4 AU) long-period comet. Currently 17th magnitude and 8.5 AU from the Sun (almost the distance of Saturn), it should brighten to magnitude 12-13 near perihelion in the summer of 2013. This comet marks the 44th (non-SOHO-STEREO) comet discovery of the year and LINEAR’s 199th comet find.

2010 BK118 was first seen back in January by the WISE (Wide Infrared Space Explorer) spacecraft. WISE is conducting a survey of the sky at 4 infrared wavelengths. Though not specifically designed to find asteroids/comets, the spacecraft observes at wavelengths optimal for finding these types of objects. Though not identified as anything special back in January, it was independently rediscovered by LINEAR last week. After a few days of observation, January’s WISE object and this month’s LINEAR object were linked as one and the same. So far there have been no reports of cometary activity even though the orbit is very cometary (long-period type). If it is truly a extinct or dormant comet than it is very large for a comet nucleus at H = 10.2 and a diameter of ~60 km. I’d be very surprised if this object doesn’t turn out to be an actual comet when larger telescopes are pointed its way.

2010 SW3 passed within 0.0058 AU of Earth on Sept. 10. This distance equals 540,000 miles, 860,000 km or 2.1 Lunar Distances. The 10 to 30 meter in diameter rock was discovered by the Mount Lemmon Survey eight days after close approach.

Asteroid    Type     MOID     a     e     i     H  Mag  Discoverer      MPEC
2010 SE12   Amor    0.115   1.36  0.18   9.3  24.3  20  Spacewatch      2010-S34
2010 SD12   Amor    0.076   1.80  0.41  23.6  23.8  20  Mount Lemmon    2010-S33
2010 SA12   Amor    0.147   2.25  0.50   6.2  20.8  20  Mount Lemmon    2010-S32
2010 SX11   Apollo  0.026   1.16  0.25   5.3  24.8  20  Spacewatch      2010-S30
2010 SZ3    Apollo  0.014   1.18  0.14   2.0  28.3  20  Mount Lemmon    2010-S26
2010 SY3    Amor    0.224   2.08  0.41   6.0  22.8  21  Mount Lemmon    2010-S25
2010 SX3    Amor    0.066   1.59  0.33   8.0  24.9  20  Mount Lemmon    2010-S24
2010 SW3    Apollo  0.0005  1.62  0.40   1.6  26.6  20  Mount Lemmon    2010-S23
2010 SV3    Apollo  0.056   1.51  0.52   6.2  20.5  19  Catalina        2010-S21
2010 ST3    Apollo  0.040   2.06  0.53   3.8  25.1  21  PanSTARRS       2010-S20
2010 SS3    Amor    0.393   1.61  0.24  26.9  20.8  21  Mount Lemmon    2010-S19
2010 SR3    Amor    0.187   1.74  0.37  12.2  21.8  21  Mount Lemmon    2010-S18
2010 SP3    Apollo  0.004   1.98  0.63   0.4  24.6  18  Catalina        2010-S16
2010 RG137  Amor    0.105   2.36  0.55   9.9  21.0  19  Mount Lemmon    2010-S15

Comet       Type       T        q     a     e     i  Mag  Period        MPEC 
C/2010 S1 (LINEAR)
            LPC    2013-05-09  4.41       1.0  126.9  17                2010-S41
2010 BK118 (discovered by WISE, rediscovered by LINEAR)
            ECC    2010-01-01  6.12 293.7 0.98 143.9  19  5030          2010-S36

Aten -  Earth crossing with semi-major axis (avg distance from Sun) < 1 AU
Apollo - Earth crossing with semi-major axis (avg distance from Sun) > 1 AU
Amor - non-Earth crossing with perihelion distance < 1.3 AU
JFC - Jupiter family comet
HFC - Halley family comet
LPC - Long-period comet
MBC - Main belt comet
ECC - Suspected extinct or dormant (or just unrecognized) comet
T - Date of Perihelion
MOID - Minimum Orbit Intercept Distance, minimum distance between asteroid and Earth's orbit
a - semi-major axis, average distance from Sun in AU (1 AU = 93 million miles)
e - eccentricity
i - inclination
H - absolute magnitude
Mag - magnitude at discovery
Discoverer - survey or person who discovered the object
MPEC - Minor Planet Electronic Circular, the discovery announcement

Around the Sky in 28 Days – Day 21

Last night, the Moon passed the bright Pleiades star cluster. Tonight the Moon passes yet another close bright star cluster, the Hyades. Both clusters are located in Taurus and though both are very close to Earth (for a cluster), they are not related.

In the chart below last night’s cluster, the Pleiades is the tight ‘dipper’-looking group of stars near the upper right part of the chart. Tonight’s Hyades cluster looks like a ‘V’ on its side (opening to the left) and is located just to the right of the Moon. This cluster is closer to us than the Pleiades (151 light-years vs 440 light-years) and older (625 million years vs 100 million years). In Greek mythology, the Pleiades were the ‘Seven Sisters’. The Hyades have a similar back story and represent 5 sisters. After the death of their brother, Hyas (hence Hyades), the sisters continually wept and the appearance of the cluster in the sky was associated with rain.

The brightest star among the group is not actually a member of the cluster. Aldebaran is the 13th brightest star in the sky and is actually located only 65 light-years from Earth, less than half the distance to the Hyades. It is a mere coincidence that the star and the cluster are located along the same line of sight. Aldebaran is a K5 orange giant star 1.7 times more massive than the Sun and 44 times larger than the Sun. It is an average magnitude of +0.9 but similar to most evolved red giant stars it’s brightness can change by a few tens of percent.

Tomorrow – Day 22 – The Moon revisits the site of a 956 year old explosion.

Around the Sky in 28 Days – Day 20

Tonight the Moon will be located near the Pleiades star cluster late this evening (from the western hemisphere).

The Pleiades is a cluster of related stars which formed out of the same molecular cloud between 75 and 150 million years ago. The cluster is slowly moving apart and may fully disperse in 250 million years. The cluster is over 1000 member stars spread over a radius of 43 light-years with most members in a inner core 8 light-years across. The total mass of the system is estimated at 800 solar masses.

The brightest star in the cluster is magnitude +2.9 with five other stars between +3.6 and +4.3. A seventh star is a variable and may have been brighter in the past (currently ~+5.1) hence why the cluster was known as the ‘Seven Sisters’ in ancient times. The brighter stars make sort of a ‘Little Dipper’ pattern and many people mistakenly think it is the Little Dipper.

Under dark skies the Pleiades look like a small dipper shaped cluster of stars about 2 Moon diameters across. Under brighter skies, the cluster may appear a small fuzzy cloud. The Moon may be too bright to see much of the Pleiades with the naked eye so use binoculars to get a better view.

Tomorrow – Day 21 – The Moon passes an even closer, bigger cluster.

Around the Sky in 28 Days – Day 18

The Moon doesn’t rise till an hour or more after twilight. Tonight marks the last night the Moon spends in a rather barren part of the sky (aside from Jupiter). Tomorrow night things get interesting again.

Don’t get me wrong. No part of the sky is truly boring. It’s just hard to find stuff when the Moon is within a few days of Full and bright. If the Moon were at only half Full there would be lots of nice double and colorful stars to see.

Tomorrow – Day 19 – the Pleiades

Introducing Comet Hartley 2

The upcoming close approach of Comet Hartley 2 should be the cometary event of the year. If the comet behaves as it did during past returns, we should expect it to be a faint naked eye comet in October and November.

Officially designated Comet 103P/Hartley, it is sometimes known by its original moniker of Comet Hartley 2. A member of the Jupiter family of comets, the comet returns to perihelion once every 6.5 years. Its current orbit takes it to within 1.059 AU of the Sun (just beyond earth’s orbit) and as far out as 5.88 AU (about 60 million miles beyond Jupiter’s orbit).

103P was first seen on photographic plates obtained on 1986 March 15, 17 and 20 by Malcolm Hartley at the Siding Spring Observatory in Australia. At the time, the comet was a faint 17th magnitude object and 9 months past perihelion. At its first two predicted returns in 1991 and 1997 the comet brightened to a nice 8th magnitude object easily visible in small telescopes and binoculars. The 2004 return was poorly placed and the comet could not be seen near perihelion. The current return, the 5th observed, is easily the best. In fact, there will be no better apparitions of Hartley 2 this century.

Before 1971, the comet was located on orbits with much larger perihelia. As a result, the comet never got bright enough to be discovered. Though the comet was probably very active during the 1973, 1979 and 1985 returns, perihelion passage occurred on the other side of the Sun and out of view from Earth. Hence the reason the comet was not seen earlier. Had Malcolm Hartley not happened upon it in 1986, the comet would have undoubtedly been found in 1991.

This year Hartley 2 reaches perihelion on 2010 October 28 at a distance of 1.059 AU from the Sun. Closest approach to Earth occurs on 2010 October 20 at 0.121 AU from Earth (11.3 million miles or 18 million km). Such close approaches by comets are uncommon though 2 have occurred in the past 15 years (C/Hyakutake in 1996 at 0.102 AU and 73P/Schwassmann-Wachmann in 2006 at 0.079 AU) and 2 more are predicted over the next 5 years (45P/Honda-Mrkos-Pajdusakova in 2011 at 0.060 AU and 209P/LINEAR in 2014 at 0.056 AU).

The comet is already a nice sight in small telescopes. The 2 images below were taken by myself with an ASA 8″ astrograph operated by Light Buckets, LLC. For more on LightBuckets and how to use their fleet of 3 Rodeo, New Mexico based telescopes (also a 24″ and 14.5″) see their site at

Many observers have reported estimates of the brightness of the comet over the past few months. Thanks to organizations like the International Comet Quarterly, Comet Observation Database and the CometObs mailing list, we are able to monitor the brightness behavior of the comet and compare it with its behavior from past returns.

The following 2 plots show the visual and CCD magnitude estimates for the current return as well as the 1991 and 1997 returns. Each is a plot of reduced heliocentric magnitude (magnitudes normalized to geocentric and heliocentric distances of 1 AU and a phase angle of 0° ). The correction to 0° was made with the comet dust phase function of Joseph Marcus. The top plot is phase-corrected heliocentric magnitude versus the log of the heliocentric distance while the next plot is versus time from perihelion.

The next plot takes a best fit of the 1997 data and converts it to the expected brightness of the comet in 2010 (after adjusting the heliocentric and geocentric distances and the phase angle). Luckily the perihelion distance in 1997 (1.032 AU) and 2010 (1.059 AU) are similar. Unfortunately the perihelion distance in 1991 was significantly smaller at 0.953 AU so it was excluded.

A quick glance at the plots show that the comet appears to be running fainter than in 1997. This was very apparent in August when the comet was almost 2 magnitudes fainter than predicted. In September it appears to have almost  “caught” up to its expected brightness. My CCD data produced magnitude of +8.1 and +8.2 on the 12th and 15th, respectively. But is this really the case? As the images earlier in the post show, this comet is very large and diffuse. My CCD images from Sept 12 and Sept 15 revealed a coma 26′ and 23′ across, respectively. The apparent shrinkage of the coma between the 2 nights may have more to do with the increasing moonlight than a real change. My CCD derived magnitudes are also running brighter than the visual estimates. Though the visual estimates are catching up with the CCD derived estimates. Most likely what is happening is that much of the very large and very low surface brightness coma is not being seen by the visual observers, and possible also not being entirely measured in the CCD images. The panel below shows how the diameter of the coma can changed just by stretching the contrast on  my Sept 15 images.

Three different contrast stretches from my Sept 15 8" LightBuckets image. Note how a change in contrast dramatically changes the apparent size of the coma. Credit: Carl Hergenrother.


The CCD observations (and perhaps some of the visual too) are also hampered by the very dense Milky Way star field the comet is currently passing through. For my observations, I re-observed each field on a later night without the comet. The brightness of the background stars in the photometric aperture were then measured and subtracted from the earlier (comet + stars) measurement. This will only become more of a problem as the comet grows larger on the sky as its continues to approach Earth.

Based on the 1997 data, the comet should reach a peak brightness of magnitude +4.5 a few days before perihelion (last week of October). Unless all current observers are underestimating the brightness of the comet (possible but, hopefully, unlikely) the comet is running a little fainter this return. Based on this a peak brightness of magnitude +5.0 might be more realistic. Still such a large diffuse comet will not be an “easy” magnitude +5.0. Unlike this summer’s Comet C/2009 R1 (McNaught) which had a very small coma and appeared star-like to the naked eye, Hartley 2 may grow to as large as a degree across. If true, the comet will only be a naked eye object to those observing from the darkest sky. For the rest of us, the comet will be a nice, large binocular object.

I’ll post updates as well as new images here and at the Cometary Science Center/International Comet Quarterly.

As always if you’d like to share your observations and/or images with my readers, send them to <> or just leave a comment on the blog.

Resources for finding the comet include:

The 103P/Hartley page at the Cometary Science Center/International Comet Quarterly.

The 103P/Hartley page at Seiichi Yoshida’s Comet pages.

A finder chart for Comet Hartley 2 can be found at Comet Chasing.

A nice collection of images can be found at the VdS-Fachgruppe Kometen (Comet Section of Germany) and Seiichi Yoshida’s Comet Homepage.

Around the Sky in 28 Days – Days 14-16

Tonight marks the Full Moon (9:18 UT, 2:18 am PDT, 3:18 am MDT, 4:18 am CDT, 5:18 am EDT). But that’s not it! Today is also the 1st day of Spring. Also both Jupiter and Uranus were at opposition only  2 days ago. As a result, the 1st Full Moon of autumn is located right next to a brilliant Jupiter and an easy to find Uranus.

The charts below show the location of the Moon, Jupiter and Uranus for the next 3 nights. Each chart is for the early evening a few hours after sunset.

Eastern sky a few hours after sunset on Sept. 22.
Eastern sky a few hours after sunset on Sept. 23.
Eastern sky a few hours after sunset on Sept. 24.


Definitely take the opportunity to use a pair of binoculars or a telescope to take a peak at Jupiter. The planet is large enough to appear as a small globe even in the smallest of instruments. Also obvious are its 4 Galilean satellites (Io, Europa, Ganymede and Callisto). The moons are surprisingly bright at 4th-5th magnitude and would be easy naked eye objects under a dark sky if they weren’t swamped by the glow of Jupiter. Note, all 4 moons may not be visible at any particular time since the moons can be located behind or even in front of Jupiter’s disk. The chart below shows the positions of the moons in the early evening of the 22nd.

Uranus is as bright as the faintest Galilean satellites and located not that much further away from Jupiter. While Jupiter is located 3.95 AU from Earth (about as close as it can get), Uranus is farther away at a distance of 19.09 AU. In small scopes, Uranus will probably look like a green star. At high magnifications one might notice the planet as a very small disk.