February 5, 2023
Sunday, February 5, 2023
How did observing Comet ZTF (C/2022 E3) turn out for you? Comet ZTF passed closest to Earth last Thursday, February 2 at a distance of 26 million miles. Comets are notoriously unpredictable in the amount of the gas and dust that they will eject as they warm up when traveling through the inner solar system. As I write this one week ago, Comet ZTF was expected to get up to 6 on the magnitude brightness scale astronomers use. A star with magnitude 6 is just barely visible to the naked eye if the sky is very dark, far from the city. A magnitude 6 comet is going to be harder to see because all of its light is spread out instead of being a pinpoint like a star.
Because the eye is more sensitive to dim light nearer the edge of the retina, sharp-eyed people would need to use averted vision instead of looking directly at the comet to see the fuzzy object. People with poorer vision like me would have no hope of seeing the comet without binoculars. Tonight the comet will be positioned about half of a fist width at arm's length north of the bright star Capella in Auriga. Tomorrow Comet ZTF will be just slightly right of Capella and the following three nights, it will skirt down the right edge of Auriga. By February 12, the comet will have reached the top horn of Taurus and on February 15, it will be just left of the orange-red eye of Taurus, the giant star Aldebaran.
The magnitude scale astronomers use to specify the brightness of something is a bit backwards—brighter objects have lower magnitudes than dimmer objects. A magnitude 6 object is barely visible with binoculars or a telescope. A magnitude 2 object is bright enough to be easily visible in even the light-polluted skies of Bakersfield. The brightest star in the night sky is Sirius, “the dog star”, at the nose or neck of Canis Major, depending on how you connect the dots. Sirius is so bright that it actually has a negative magnitude of -1.46!
You can find Sirius by extending the line of Orion's belt stars (they have magnitudes between 1.7 and 2.2) down to the left to the first star you see on that line. Sirius is the nearest star outside of the solar system that we in Kern County can see without a telescope. (Those south of 30 degrees North latitude will be able to see Alpha Centauri, the actual closest star system outside of the solar system.) Sirius appears so bright in our sky partly due to its proximity to us at just 8.7 light years away. Sirius also emits about 25 times as much light as the sun, so that coupled with its close distance makes it very bright in our sky.
Last night the moon was at full phase. On the magnitude scale, a full moon comes in at -12.7. Tonight will be one night into the waning gibbous stage, ending when it is at third (or last) quarter phase on the night of February 12/13. At the quarter phase, the moon is at magnitude -12.1 but that's still plenty bright enough to wash out many of the fainter stars around it.
A thin crescent moon is just at magnitude -9.5. In a manner that is sort of like the Richter scale of earthquakes which increases by powers of ten, the magnitude scale of astronomy goes by powers of 2.5, so a thin crescent moon is about 7.6 times dimmer than a full moon (you subtract the magnitude numbers: 12.7-9.5 = 2.2 and then raise 2.5 to that power 2.2 to get 7.6 times dimmer in actual intensity).
The full moon and waning gibbous moon will make it impossible to see Comet ZTF without binoculars. The comet is also getting farther from the sun and Earth with each night. Comet ZTF came from the Oort Cloud many thousands of times farther away from the sun than is Pluto taking a path almost perpendicular to the orbital planes of the planets. It came from above the planet orbits and it will cross the Earth's orbit plane (the ecliptic) on February 12 heading down below. As the comet came in close to the sun, the gravity of the planets adjusted its orbit so that the orbit is now unbound and the comet will never return. In several hundred thousand years from now, it may wander into another planetary system.
In our evening sky, you'll see super-bright Venus (at magnitude -3.9) low in the southwest and climbing higher with each succeeding night as it inches toward a rendezvous with -2.2 magnitude Jupiter on March 1. Saturn is now lost in the evening twilight glow. At magnitude -0.1, Mars is still plenty bright, outshining all of the stars of Taurus in which Mars will be found for the next couple of months.
I hope that you'll be able to find a time and place sometime to gaze up in wonder at a dark night 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
