Here are this week’s best astronomical events that you can view right from your backyard. Below is a spectacular image of the Horsehead Nebula, named so because of its resemblance to a horse’s head from the Earth. More information here if you are interested! As for events this week, the December solstice happens next Tuesday. This means for those of us in the Northern Hemisphere that it will be the shortest day and longest night of the year. So for anyone else who hasn’t been a fan of it getting dark so early in the evenings, this is the turn around point for us! At any rate, I hope you find some time to observe the stars this week, perhaps with some hot chocolate and a warm blanket to bundle up in. Happy exploring!
“Past time is finite. Future time is inifinte.” –Edwin Hubble, namesake of the Hubble Space Telescope
You can use the star chart provided here. And for any terms or units you may be unfamiliar with, I’ve provided a glossary at the end of the guide for your convenience. All events were calculated based on Boulder, CO (latitude 40⁰ N), but should provide good approximations for most cities near the same latitude. If you would like, you can change your location on the linked sites for each event. Let me know if you have any questions. I’d love to know what you see this week!
Thursday 12/17
07:05 MST Iridium Flare (Iridium 19)
The Iridium 19 satellite will cause an iridium flare of magnitude -6.3 that will be seen as a bright flash across the sky. The flare will begin at an elevation of 72⁰ in the NNW. More information here.
Friday 12/18
16:54 MST Moon at First Quarter
The Moon will be at first quarter tonight. The different Moon phases occur because of our changing perspective as the Moon orbits the Earth. The phases cycle every 29.5 days–from new moon, to first quarter, to full moon, to last quarter, and then finally to new moon again. The half phase is ideal for observing the craters and mountains on the Moon’s surface. More information here.
Saturday 12/19
16:57 MST Moon-Uranus Conjunction
The Moon and Uranus will be very close, passing within 1⁰06’ of each other. The Moon will have a magnitude of -12.3, while Uranus will have a magnitude of 5.8. Both will be located in the constellation Pisces. From Boulder, the pair will become visible around 16:57 MST as the dusk sky fades, 44⁰ above the SE horizon. The pair will reach its highest point 55⁰ above the southern horizon around 19:08 MST, and will remain visible until 00:50 MST when they sink below the western horizon. More information here.
Sunday 12/20
17:30 MST Iridium Flare (Iridium 55)
The Iridium 55 satellite will cause an iridium flare of magnitude -5.3 to -5.7 that will be seen as a bright flash across the sky. The flare will begin at an elevation of 15⁰ in the West. More information here.
17:39 MST Iridium Flare (Iridium 95)
The Iridium 95 satellite will cause an iridium flare of magnitude -2.8 to -5.5 that will be seen as a bright flash across the sky. The flare will begin at an elevation of 13⁰ in the WNW. More information here.
18:15 MST Iridium Flare (Iridium 5)
The Iridium 5 satellite will cause an iridium flare of magnitude 0.3 to -7.3 that will be seen as a bright flash across the sky. The flare will begin at an elevation of 29⁰ in the South. More information here.
Monday 12/21
14:11 MST Moon at Perigee
The Moon will reach the closest point in its orbit to the Earth. The Moon’s distance from the Earth varies because its orbit is not perfectly circular. Instead, the Moon traces out an ellipse with Earth at one of its foci. The Moon’s distance varies by 10% throughout its orbit (between 363,000 km and 405,000 km), which means its size in the night sky varies by nearly 13%. Tonight the Moon will appear to have an angular size of 32.42′, as compared to its average angular size of 31.07′. It should also be noted that the Moon’s apparent size in the sky will also vary depending on its vicinity to the horizon. For instance, the Moon will appear much larger when it is closer to the horizon. More information here.
Tuesday 12/22
24:00 MST Ursid Meteor Shower
The Ursid meteor shower will reach its peak activity tonight. Shooting stars associated with the shower should be visible from December 17th through December 25th. The maximum rate of visible meteors is expected to reach 10 per hour; however, this zenithal hourly rate is dependent on many factors, such as sky conditions, light pollution at the observer’s location, and altitude of the meteor shower’s radiant in the sky. The Moon will be 11 days old at the peak, and so will present significant interference in the early evening sky. At midnight, the shower’s radiant will appear 33⁰ above the northern horizon, with the meteors traveling directly outward from this point. In order to see the most meteors possible, it is best to look at any dark patch of sky 90⁰ around the radiant, rather than at the radiant itself. This is because the meteors will typically appear brightest at these locations. It is so named the Ursid meteor shower because of its radiant’s location within the Ursa Minor constellation (aka Little Dipper). More information here.
21:48 MST December Solstice
The winter solstice happens at the same instant for all of us, everywhere on the Earth! The solstice occurs when the sun in our sky reaches its farthest southward point for the year. In the Northern Hemisphere, we experience the winter solstice as the shortest day and longest night of the year. More information here.
Wednesday 12/23
17:20 MST ISS Passing
The ISS will make a visible pass (magnitude -3.4) starting at 10⁰ elevation in the NW and moving ESE, reaching a highest elevation of 73⁰. More information here.
Glossary
Apogee: The point in the orbit farthest away from the Earth.
Apehelion: The point in the orbit farthest away from the Sun.
Arcminute: An angular measurement, defined as 1/60 of one degree. Denoted by ‘. (approx. 1 inch at a distance of 100 yards, the Moon is approx 31’)
Arcsecond: An angular measurement, defined as 1/60 of an arcminute. Denoted by “. (approx. dime at a distance of 1 mile )
A great resource for understanding and approximating angular measurements in the night sky, here. The Moon is approx 1/2⁰. Your outstretched hand is approx 20⁰, with each finger approx 1⁰. Your closed outstretched palm is approx 10⁰.
AU: Astronomical Unit, Defined as the distance between the Sun and Earth.
Conjunction: An alignment of two celestial bodies such that they present the least angular separation as viewed from Earth.
Elongation: The angle between the Sun and a planet, with Earth as the reference point. The greatest elongation of a planet occurs when this separation angle is at its largest. More information here.
Iridium Flare: Iridium fares occur when sunlight is reflected off the antenna (of one of the 66 active telecommunication sats in LEO, known as the Iridium constellation) directly down at Earth. This reflection causes an illuminated spot on the surface of the Earth. To an observer on the ground, the event appears to be a bright flash, or flare in the sky, which lasts for a few seconds.
ISS: International Space Station. More information here.
Magnitude: A logarithmic measure of the brightness of an object. Brighter objects have a lower (more negative) magnitude. More information here.
Opposition: When a celestial body is opposite the Sun in the sky.
Perigee: The point in the orbit closet to the Earth.
Perihelion: The point in the orbit closest to the Sun.
Radiant: The point in the sky where meteors of a meteor shower appear to originate.
Time Conversions from MST
Eastern Daylight Time, EST = MST + 2:00
Central Daylight Time, CST = MST + 1:00
Mountain Daylight Time, MST
Pacific Daylight Time, PST = MST – 1:00
Alaska Daylight Time, AKST = MST – 2:00
The standard for astronomical times is UTC if you happen to come across it, where MST = UTC – 6:00.
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