May 06 – Daphnis

Today in 2005, Saturn’s moon Daphnis was discovered by the Cassini team.

Daphnis (image credit: NASA)
Daphnis (image credit: NASA)

Don’t be fooled by the large object at the bottom of the photograph: that’s Epimetheus, a moon about twelve times wider than Daphnis.  The birthday boy himself is a tiny, barely discernible dot towards the centre of the frame.  You might be surprised by the use of the word “boy” there, assuming Daphnis might be named after a Daphne.  But it wasn’t.  Daphnis was a shepherd (this is a shepherd moon), a very good friend of Pan, and the inventor of pastoral poetry.

Apollo and Daphnis (Perugino).
Apollo and Daphnis (Perugino).

The moon Daphnis, being ridiculously small, wasn’t an easy target to photograph. Fortunately, though, its position in the Keeler Gap within Saturn’s A ring gave it away by causing ripples within the ring. In order to stay within the Keeler Gap, Daphnis has to follow a near-perfect circular orbit. The difference between Daphnis’ perikrone and apokrone (closest approach to and furthest distance from Saturn) is only about 9 km (under 6 miles).

 1896  –  Asteroid 417 Suevia discovered by Max Wolf. Suevia is a K-type/S-type main belt asteroid of approximately 41 km (25.4 miles) diameter. it sweeps serenely round the Sun in an orbit ranging from 363 to 474 billion miles.

May 05 – 70 Panopaea

Asteroid 70 Panopaea was the last to be discovered by our old friend Hermann Goldschmidt at his Paris Observatory on May 5th 1861.  It is a large, dark C-type main belt asteroid about 75 miles in diameter.

Panopaea was a water nymph. She was one of the 50 daughters of Nereus and Doris known as the Nereids (well, I say 50, but if you start counting them up in Homer and Hesiod you end up with about 90). They are particularly associated with the Aegean Sea.

Finding a picture of Panopaea is proving difficult, so here is her father, Nereus.

Herakles and Nereus
Herakles and Nereus

Today’s photograph shows a detail from a Greek black-figure lekythos (oil storage jar) in the Louvre, Paris, decorated by the so-called Istanbul Painter, although this jar was found in Boeotia, southern Greece.  Herakles (Hercules if you prefer) is shown holding onto Nereus in an attempt to extract from him the location of the Apples of the Hesperides.

1853  –  Asteroid 26 Proserpina discovered by Robert Luther, and named after a Roman goddess associated with fertility and agriculture. Proserpina is about 90 km (56.5) km across (possibly a little more) and is an S-type asteroid. It’s a fairly cold place, in common with the rest of the main belt, with a surface temperature of 166 Kelvin (minus 157° Celsius).

May 04 – 416 Vaticana

Happy Star Wars Day.

1886  – on May 4th 1866, asteroid 258 Tyche was discovered by Robert Luther. It is a 650m wide “S” type main belt asteroid, and is possibly a member of the Eunomia family. It was named after the Greek goddess whose Roman equivalent was Fortuna. Her main responsibility was to decide how prosperous a city should be.

1896  –  Asteroid 416 Vaticana was discovered on May 4th, 1896, by Auguste Charlois. It is an S-type main belt asteroid, about 85 km across, and was named after the Vatican Hill (Mons Vaticanvs) in Rome.

1989 – Launch of shuttle mission STS-30 carrying the Magellan probe, aka the Venus Radar Mapper, to be sent on its way to Venus. This was the first ever launch of a spacecraft from a shuttle. Magenta arrived at Venus on August 8th, 1990, and used a high-gain parabolic antenna to allow it to map the Surface through the opaque Venusian atmosphere. The probe burned up in this atmosphere on October 13th, 1994. Being mostly composed of carbon dioxide (96.5%) and nitrogen (3.5%), both of which are colourless, you might think that seeing the surface from orbit would be a doddle, but it is the presence of clouds of sulphuric acid, blowing around the planet at over 200 mph (about 300 km/h) that pose the problem.


The photograph shows the Venusian volcano Sapas Mons, a monster by Earth standards at over 240 miles wide and nearly a mile high.


May 03 – Messier 3

Messier 3, a globular cluster in the constellation Canes Venatici (the hunting dogs) was discovered by Charles Messier on May 3rd, 1764, but was not actually resolved into a collection of stars for another 20 years (by William Herschel).

Messier 3. Image credit: Hewholooks (Hunter Wilson).
Messier 3. Image credit: Hewholooks (Hunter Wilson).

As one of the brightest clusters (apparent magnitude 6.2) it is a popular target for amateur observers. Messier 3 contains about 500,000 stars, and is approximately 33,900 light years from Earth. Over 270 of those stars are variables, a much higher number than the average cluster; and of those, 133 are RR Lyrae variables: old, small-mass stars (about half the size of the Sun) which can be used to measure astronomical distances using the inverse square law.

1888  –  Main belt asteroid 277 Elvira was discovered by Auguste Charlois on May 3rd, 1888. It is approximately 27 km in diameter (not that I’m suggesting we are talking about a perfect sphere here) and is a member of the Koronis family. The origin of the name Elvira is not certain, but might have been inspired by a character in the Méditations poétiques of Alphonse de Lamartime, written in 1820.
The Koronis family comprises at least 300 members, mostly under 20 km wide, all following each other around in a similar orbit, and thought to exist as a result of some massive collision over two billion years ago.


May 01 – Nereid

Neptune’s third largest moon, Nereid, was discovered on May 1st, 1939, by Gerard P Kuiper, using the 83-inch Otto Struve Telescope at Mcdonald Observatory, Texas. It is about 170 km in diameter, and has a day lasting just 11.5 hours, but a year almost identical to ours, at 360 days.

Not Voyager 2's greatest photograph, but the best we have of Nereid (image: NASA).
Not Voyager 2’s greatest photograph, but the best we have of Nereid (image: NASA).

Nereid has a prograde rotation, and a very eccentric orbit, which takes it from 853,000 miles at periapsis to 5,999,000 miles at apoapsis. This has led astronomers to believe it is either a captive asteroid (or Kuiper Belt Object) or has had a previously more normal orbit changed by the capture of Triton.

The Nereids after whom this moon is named were the fifty daughters of Nereus and Doris. They were sea nymphs who attended Neptune (he married one of them, Amphitrite, who bore his son Triton.