April 16, 2023

Mid-April at 8:30 PM looking west-southwest

Sunday, April 16, 2023

In this evening's sky shortly after sunset, you'll see the three planets closest to us making a line on the sky in the southwest-west: Mercury closest to the western horizon, then super-bright Venus, and then Mars, highest up. Mercury is fading as it drops back toward the sun but it's still one of the brighter points in the sky. Mars is also fading but not as quickly as Mercury is. Mars is also still one of the brighter points in the sky, shining brighter than the stars of Gemini in which Mars is now moving through. Mars will be in Gemini for about another month or so. 

Three famous star clusters are also on that same line with the three planets: the Pleiades in Taurus, the Beehive Cluster in Cancer, and the Hyades that make up most of the V-nose of Taurus (the bright star Aldebaran at one end of the V is not part of the cluster). Venus is a little left of the Pleiades. In the attached star chart, I also show the “ecliptic” which is the projection of Earth's orbit onto the sky and also the path that the sun appears to move on throughout the year. Since all of the planet orbits are closely aligned with each other (so a sideview of our solar system would look pretty flat), the three planets are close to that ecliptic arc across the sky. It's a nice coincidence that we also happen to have three beautiful open clusters near the ecliptic as well. There are many other star clusters farther from the ecliptic. 

The moon is now a waning crescent visible in the east just before sunrise. This morning it was next to Saturn. On April 20, the moon will be at new phase and lined up with the sun to make a solar eclipse for those on the other side of the world in Australia and Indonesia. On April 22, the moon will be a thin waxing crescent between Venus and the Pleiades low in the west after sunset. 

The night of April 22/23 is the peak of the Lyrid meteor shower made as Earth plows through the dust trail left behind by Comet Thatcher. The dust bits hit the upper atmosphere at 30 miles/second and quickly burn up. With the moon setting earlier in the evening, you should be able to see the fainter meteors outside of the light-polluted skies of Bakersfield.

One interesting news item is about the damage that happens when people use a magnet to test if a rock is a meteorite. Many meteorites are magnetic and most Earth rocks are not, so people will often use an ordinary hand magnet to see if a rock is a meteorite. Unfortunately, it's not a foolproof way to distinguish meteorites from Earth rocks because there are some types of Earth rocks that do contain iron and are magnetic (or iron slag from steel manufacturing) while the meteorites that are from giant impacts on the moon and Mars are not magnetic.

The damage comes from the ordinary hand magnets, which are thousands of times stronger than Earth's magnetic field, easily wiping away the vestigial magnetic signal of the parent body of the chip falling to Earth as a meteorite. For example, the black chunks of meteorites strewn across a section of western Sahara were found to contain crystals that formed on Mars over 4.4 billion years ago when Mars still had its global magnetic field. Most of the martian meteorites are from younger parts of Mars, forming after Mars had lost its global magnetic field, so most martian meteorites are not magnetic. However, this set of meteorites, called “Black Beauty”, would have been magnetic, showing us the signal from Mars' ancient magnetic field. Unfortunately, somewhere along the chain of ownership from its find in 2011 to the laboratory, a hand magnet had been brought near or in contact with the rock chunks and realigned the magnetic direction. 

A better way than ordinary hand magnets is to use a “susceptibility meter” that has a much weaker magnetic field. Unfortunately, a susceptibility meter is pretty pricey at a few thousand dollars. 

One last astronomy item is the Astronomy Picture of the Day I showed my astronomy students on April 10 (see apod.nasa.gov). The APOD was the “Running Chicken Nebula”, so called because it's supposed to look like a chicken running across the sky if you have a great imagination (or have ingested the right sorts of pharmaceuticals). In the middle of the picture are some tiny black splotches that are molecular clouds. The APOD image is linked to a high-res image over 9000 pixels across.

The nebula is about 100 light years across. The smallest of the dark molecular clouds visible in the high-res image are a few pixels across. Well, it turns out that a single pixel at the scale of image is about the size of the diameter of Pluto's orbit (see the perspective image at bottom). Our solar system is tiny! It's inside of the dark molecular clouds that stars and planets form. The Running Chicken Nebula is just one of many thousands of emission nebulae in our galaxy.

I hope you'll be able to experience in the near future, a truly dark sky filled with thousands of stars!

Nick Strobel
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com

Size scale perspective using the Running Chicken Nebula