Ursids and Solstice and Last Quarter

The Ursids, of the night of December 21/22, are the last noticeable meteor shower of the year.

Ursid meteors

The expected date of this shower’s modest peak is just a day after the midwinter solstice (of Dec. 21 at 10:44 Universal Time).  And whereas the Geminids of Dec. 13/14 coincided with Full Moon, the Ursids come a day and a quarter after Last Quarter (Dec. 21, 1:56 UT).  So the Moon, with its glare less than half as strong as when it was Full, will not rise into the sky till about an hour after midnight.

These meteors, also called Ursa-Minorids, appear to radiate from the constellation of the Little Bear, which has the North Pole Star at one end and Kokab at the other.  So this radiant is in the sky all night, swinging around the Pole and becoming highest at the night’s end.

The Ursid meteors are particles following roughly in the orbit of the comet from which they crumbled: 8P Tuttle (the “P” denotes “periodic”), which last came by in January 2008 and is due next in August 2021.  The orbit drops steeply from the north across the December part of Earth’s orbit, and that is why these midwinter meteors appropriately come to us from the Arctic of the sky – arktos is Greek and ursa Latin for “bear.”

Ursod meteors striking Earth

They will probably be more of a light snow-flurry than a shower.  At their peak, and under ideal conditions, only around 10 per hour may be seen.  Yet there have been occasional Ursid storms, even to more than 100 per hour, as in 1993.

The rate is usually above half maximum for about half a day, and the peak this year is expected to be about 9 Universal Time on Dec. 22, which is convenient for America – 4 AM in the Eastern time zone, 3 in the Central, 1 on the west coast.  Meteor streams can be clumpy and braided, and there could be subsidiary peaks in the two following nights.

Sky & Telescope has a good piece on the Geminids and Ursids in the December issue (pages 48-49).  I notice that they’re now copying my style of representing meteor radiants in charts – well, you can’t copyright such things, and it won’t be the first time we’ve borrowed ideas from each other.  I’d be interested to know whether the meteor streaks are drawn by hand in their art department or programmed, as I do, to relate to the strength of the shower and the nearness in time of the picture to the shower’s peak.

The Moon, before it reached Last Quarter, passed Regulus on Sunday Dec. 18, at 18 UT, so closely that it occulted the “Little King” star – but only as seen from the southernmost coast of Australia.


8 thoughts on “Ursids and Solstice and Last Quarter”

  1. Coming to this a little late, but…

    The main part of the velocity at which a meteoroid enters the Earth’s upper atmosphere is a combination of the Earth’s own orbital motion and that of the meteoroid particle’s. The Earth’s orbital velocity averages 30 km/sec, while orbital velocities for Solar System meteoroids in near-Earth space are around 42 km/sec. This gives a range of observed meteor velocities from those objects approaching us from directly behind (so in effect from opposite the EDOT; essentially they are almost in the Earth’s own orbit) to those from directly ahead (essentially from the EDOT position), of between about 11 to 72 km/sec. Earth’s own gravity increases the approach velocity by around 3 km/sec or a little more.

    However, as Guy already noted, there are two additional factors to remember.

    One is that this velocity range refers to the meteoroids as they enter the Earth’s atmosphere and begin to glow as meteors. As soon as they do so, other forces come into play, most notably atmospheric drag, which begins to reduce that initial speed as the particle encounters greater atmospheric densities with increasing depth, if not necessarily in a readily-predictable way. This is because as the object begins to glow and shed the outer layers of its substance, it can create jet effects in different directions dependent on things such as the chemistry and physical construction of the particle, as well as its size, shape, surface approach angle, whether and how it is rotating. Sometimes the particle can fragment, perhaps catastrophically so, and those are times when the deceleration really becomes apparent to a visual observer – assuming you’re lucky enough to catch the event in your central vision, at least!

    The second is that although we tend to cite a, sometimes very specific, atmospheric entry velocity for meteors in each shower, the individual particles forming the meteoroid stream in space all have velocities that are slightly different to one another, because they are not all on exactly identical orbits. Partly this is because the body they originated in was not a single point, but an object (generally a comet at some time, in all probability) with its own physical dimensions, partly because the meteoroids have left that body at different times in its orbit and likely in different ways, and partly because as soon as they leave that body and become free-orbiting objects on their own, they are subject to other forces within the Solar System. Of these latter, gravity is the obvious one, but solar radiation (including heating and cooling, and for the smaller dust particles, the simple impact of electromagnetic radiation) and physical impacts with other particles play their roles too. So the range of atmospheric-entry velocities in a given meteor shower is only “about” the value as stated, although the actual variation is much harder to estimate, if still probably within a few km/sec of the “mean”, if we call it that.

  2. Thanks for the post about the Ursid meteors! I was out last night taking a few pictures of the planets among the constellations, and as I was lining up to take a picture of the area of Pisces in which Uranus can currently be found, I saw a bright meteor streak vertically down through the area and continue south. I judged that the direction was very close to celestial north to south, so it probably was an Ursid. I did manage to get a shot of Uranus, and it amazes me that even with a 30 second image of the sky and only a mild telephoto lens, the camera records the distinctive color of Uranus as shown in this blow-up of my image:
    Venus, Mars, and Neptune were also out last night. Mercury was too but I missed him!

  3. If I understand correctly, the angle between a meteor stream direction of travel and EDOT is the main determinant of the speed of the meteors. Meteors like the Ursids that are coming in sideways to our orbit are therefore of medium speed.

    Does the speed of comets (and comet debris) have a noticeable effect on the speed of meteors (in km/sec) ?

    1. Certainly, closeness of the radiant to the EDOT (Earth’s direction of travel) is a factor: if the radiant is at the EDOT, the mereors are hitting Earth from dead ahead. And so yes, the Ursids and Geminids both have medium speed because they come in roughly perpendicular to our orbit. It’s not the only factor, because another is the speed of the meteors at this point in their elliptical or parabolic orbit. If a body’s orbit is very eccentric, its speed varies greatly, slow at aphelion, fast at perihelion, so meteors (or comets) near the inward end of such orbits are moving fast. I now make my calculations for a comet’s ephemeris or phenomena spit out the speed (relative to the Sun) at the date. But whether this factor is more or less than the other one I don’t know. Also, meteors slow greatly as they enter our atmosphere; the speeds cited for them refer to speed in the atmosphere, and are “typical”, but the speed of an individual meteor must also be affect by its size and by the angle at which it enters the atmosphere; and it slows as it gets deeper in. Alastair McBeath would know more.

      1. As you stated, the orbital speed in an elliptical orbit is faster at perihelion. As you know, the orbital speed is also affected by the diameter of the comet’s orbit (the orbital speed of Mercury averages 47 km/sec., Venus is @ 35, Earth @ 29 km/sec.)

        I suspect that the location of the radiant as compared to EDOT would be the major factor in a meteor’s speed, with the orbital speed of the debris being negligible in a meteor’s atmospheric speed in km/sec. (Though I haven’t calculated it algebraically, I would guess that the orbital speed would be fairly uniform when a body’s distance from the sun is 93 million miles , regardless if the orbit is parabolic or circular.)

        Thanks for pointing out the decrease in speed as the meteor gets deeper in the atmosphere. I’l look for the atmospheric drag the next time I observe a meteor.

  4. Funny that there is not phase of the moon called ‘half’. First and last quarters show it only as such, but the real Half Moon was the ship Henry Hudson sailed up the Hudson River. A park in Riverdale is named after it and once when I was on assignment there I looked up and saw a last quarter moon and felt how appropriate I was there just then. Next is how Staten Island got its name.. Well when Peter Stuyvesant was approaching America’s shores, it was quite cloudy as as he peered through the fog he thought he spotted land. ” ‘s dat an island?” he asked…. Ta-da-dum…..

    1. Following the pattern of a “quarter” moon, a full moon should be called a half moon.

      1. There’s probably no hope of reaching agreed consistency in the naming of the Moon’s phases.
        “Full Moon” (used by all) and “Half Moon” (used popularly) are based on illuminated area as seen from Earth.
        “First Quarter” and “Last” or “Third Quarter” are based on time – the fraction of the lunation time – though they could also be said to be based on the visible illuminated area as a fraction of the circumference.
        “Waxing Half Moon” and “Waning Half Moon” would extend the visible-illuminated-area to all, but probably would not catch on.
        Though “Quarter” and “Full” seem inconsistent (seeming to mean 1/4 and 1/1), a defence of the “Quarter” usage is that the quarter Moons are indeed more like a quarter as bright than half as bright as the Full phase.

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