Extreme Gamma Rays

The highest-energy gamma ray photon ever recorded was recently observed by the Large High Altitude Air Shower Observatory (LHAASO) on Haizi Mountain, Sichuan province, China, during its first year of operation.

1.42 ± 0.13 PeV

That is 1.4 petaelectronvolts = 1.4 × 1015 eV! The origin of this fantastically energetic photon hasn’t been localized, but possible candidates are the Cygnus OB2 young massive cluster (YMC), the pulsar PSR 2032+4127, or the supernova remnant candidate SNR G79.8+1.2.

The LHAASO observatory, in China, observes ultra high-energy light using detectors spread across a wide area that will eventually cover more than a square kilometer. Institute of High Energy Physics/Chinese Academy of Sciences

How much energy is 1.4 PeV, actually?

We can calculate the frequency of this photon using

\textup{E}=h\nu


where
h = Planck’s constant = 4.135667696 × 10-15 eV·Hz-1
ν = the photon’s frequency
E = the photon’s energy

Solving for ν, we get

ν = 3.4 × 1029 Hz

Next, we’ll calculate the photon’s wavelength using

c=\lambda \nu

where
c = the speed of light = 299792458 m·s-1
λ = the photon’s wavelength

Solving for λ, we get

λ = 8.9 × 10-22 m

To give you an idea of just how tiny 8.9 × 10-22 meters is, the proton charge radius is 0.842 × 10-15 m, so 1.9 million wavelengths of this gamma ray photon would fit inside a single proton! An electron has an upper limit on its radius—if it can be said to have a radius at all—between 10-22 and 10-18 m. So between 1 and 2000 wavelengths of this gamma ray photon would fit inside a single electron.

Using Einstein’s famous equation E = mc2 we can find that each eV has a mass equivalent of 1.78266192 × 10-36 kg. 1.4 PeV then gives us a mass of 2.5 × 10-21 kg. That may not sound like a lot, but it is 1.5 million AMUs (Daltons), or a mass comparable to a giant molecule (a protein, for example) containing ~200,000 atoms.

This and other extremely high energy gamma ray photons are not directly detected from the Earth’s surface. The LHAASO detector array in China at 14,500 ft. elevation detects the air shower produced when a gamma ray (or cosmic ray particle) hits an air molecule in the upper atmosphere, causing a cascade of subatomic particles and lower-energy photons, some of which reach the surface of the Earth. It is the Cherenkov photons produced by the air shower secondary charged particles that LHAASO collects.

References
Conover, E. (2021, June 19). Record-breaking gamma rays hint at violent environments in space. Science News, 199(11), 5.
https://www.sciencenews.org/article/light-energy-record-gamma-ray

Z. Cao et al. Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sourcesNature. Published online May 17, 2021. doi: 10.1038/s41586-021-03498-z.

We’re on a Collision Course with a Gas Cloud

Smith Cloud

A giant cloud of mostly hydrogen gas with enough material to make over a million suns is heading towards our Milky Way at a speed of 45 miles per second. Called the Smith Cloud (after Gail Bieger-Smith who discovered it in 1963), this 9,800 × 3,300 ly high velocity cloud (HVC) is about 40,000 ly distant and is expected to slam into our Milky Way galaxy in about 27 million years, causing the birth of many new stars a quarter-way round the galaxy from us.

Smith Cloud is located in the constellation Aquila, the Eagle

The Smith Cloud is located in the constellation Aquila, and has an apparent diameter around 11° across its long axis. It is only visible using radio telescopes (spin-flip transition of neutral atomic hydrogen), or by detecting hydrogen absorption lines Doppler shifted and superimposed upon the spectra of more distant stars that are shining through the cloud.

The origin of the Smith Cloud is unknown. It may have originated within the Milky Way galaxy itself, or it may be extragalactic. The upcoming collision may not be the first time the Smith Cloud has encountered the disc of the Milky Way. It may be embedded in a large halo of dark matter which would have kept the cloud from being completely disrupted during any past encounters.

The Smith Cloud is a great example of an object that would never have been discovered were it not for radio astronomy. Felix J. Lockman, who has published extensively on the Smith Cloud, has created Radio Astronomy: Observing the Invisible Universe for The Great Courses. Dr. Lockman’s engaging lecture style, his clear explanations, and thorough knowledge of the subject matter makes this the perfect introduction to the subject. Highly recommended!

Incidentally, Jay Lockman discovered a region in Ursa Major that is relatively free of neutral hydrogen gas and dust, thus affording a clearer view into the distant universe. It is named, appropriately, the Lockman Hole.

References

Alig, C. et al. “Simulating the Impact of the Smith Cloud.” The Astrophysical Journal 869 (2018): 1-6.
arXiv:1901.01639 [astro-ph.GA]

Hu, Y. et al. “Magnetic field morphology in interstellar clouds with the velocity gradient technique.” Nature Astronomy (2019): 1-7.
arXiv:2002.09948 [astro-ph.GA]

Lockman, F.. “Accretion Onto the Milky Way: The Smith Cloud.” Proceedings of the International Astronomical Union 11 (2015): 9 – 12.
arXiv:1511.05423 [astro-ph.GA]

Star Stuff

The elements that make up the stars also exist here on Earth. In fact, our Earth, and indeed all the planets, were created from the dust and gas produced by previous generations of stars that existed before our Sun and solar system formed. We truly are made of stardust!

Stars are made up almost entirely of hydrogen and helium. Here is a table of the most abundant elements in our Sun.

Element% by atoms
Hydrogen92.2%
Helium7.7%
Oxygen0.0473%
Carbon0.0272%
Neon0.0130%
Nitrogen0.0065%
Magnesium0.0033%
Silicon0.0030%
Iron0.0028%
Sulfur0.0013%
Most abundant elements in the Sun

It is not a trivial matter to determine the abundance of elements in the Sun. For most elements, astronomers have to look at the strength of spectral absorption lines in the photosphere. Some elements, like fluorine, chlorine, and thallium, require looking for their spectral lines inside of sunspots, which are cooler-than-average regions of the photosphere. Other elements require that we look at spectral lines in the solar corona, or capture and analyze the solar wind. And some elements we are simply unable to detect.

The region of the photosphere that is amenable to spectral study represents only about 2% of the mass of the Sun. Since the Sun’s formation 4.6 Gyr ago, some gravitational settling of heavier elements and diffusion of hydrogen towards the surface means the Sun is not uniform in composition. Fortunately, the relative abundances of the elements heavier than helium are probably similar throughout the Sun.

Lithium, the third element in the periodic table after hydrogen and helium, is the odd element out. It has a relative abundance in the solar photosphere that is only 1/170th that found in meteorites. The Sun’s original supply of lithium has largely been destroyed by the high temperatures inside the pre-main-sequence Sun, and today at the hot bottom of the Sun’s convection zone.

Light pollution is a problem here on Earth, but on the Sun we have a problem with “line pollution”. There are so many spectral lines that the weak signatures from some elements become difficult or impossible to isolate and measure. There is much blending of overlapping lines, and some elements—most notably iron which is the ninth most abundant element in the Sun—are “superpolluters” with hundreds to thousands of spectral lines from both excited and ionized states.

Sometimes, the spectral lines of interest are in a region of the electromagnetic spectrum (ultraviolet, for example) that can only be observed from space, and that creates additional challenges.

Notably, the noble gases helium, neon, argon, krypton, and xenon have no photospheric absorption lines that can be observed, and we must look to coronal sources such as the solar wind, solar flares, or solar energetic particles for information about their abundances.

Helium—the second most abundant element in the Sun—requires an indirect approach combining a theoretical solar model and observational helioseismology data to tease out its abundance.

The following elements are undetectable in the Sun: arsenic, selenium, bromine, technetium, tellurium, iodine, cesium, promethium, tantalum, rhenium, mercury, bismuth, polonium, astatine, radon, francium, radium, actinium, protactinium, and all the synthetic elements above uranium on the period table.

Interestingly, helium was discovered in the Sun before it was discovered on Earth! That’s why this element is name after Helios, the Greek god of the Sun.

The energy source that allows stars to shine steadily, often for billions of years, is fusion. Fusion in a star can only occur where both the temperature and pressure are very high. Usually (but not always!), this occurs in the core of the star. When the element hydrogen fuses into helium, a huge amount of energy is released in the process. Lucky for us, fusing hydrogen into helium is difficult to do in a one-solar-mass star. On average, any particular hydrogen atom in our Sun has to “wait” about five billion years before having the “opportunity” to participate in a fusion reaction!

In order for sustained fusion to occur in the core of a star, the star must have sufficient mass so that the core temperature and pressure is high enough. Present thinking is that the lowest mass stars where sustained fusion can occur have about 75 times the mass of Jupiter, or about 7% the mass of the Sun.

References

Lodders, K. 2020 Solar Elemental Abundances, in The Oxford Research Encyclopedia of Planetary Science, Oxford University Press
arXiv:1912.00844 [astro-ph.SR]

The Early Radio Universe

As the expanding universe cooled, the first neutral1 hydrogen atoms formed about 380,000 years after the Big Bang (ABB), and most of the hydrogen in the universe remained neutral until the first stars began forming at least 65 million years ABB.

The period of time from 380,000 to 65 million years or so ABB is referred to as the “dark ages” since at the beginning of this period the cosmic background radiation from the Big Bang had redshifted from visible light to infrared so the universe was truly dark (in visible light) until the first stars began to form at the end of this period.

All the while, neutral hydrogen atoms occasionally undergo a “spin-flip” transition where the electron transitions from the higher-energy hyperfine level of the ground state to the lower-energy hyperfine level, and a microwave photon of wavelength 21.1061140542 cm and frequency 1420.4057517667 MHz is emitted.

Throughout the dark ages, the 21 cm emission line was being emitted by the abundant neutral hydrogen throughout the universe, but as the universe continued to expand the amount of cosmological redshift between the time of emission and the present day has been constantly changing. The longer ago the 21 cm emission occurred, the greater the redshift to longer wavelengths. We thus have a great way to map the universe during this entire epoch by looking at the “spectrum” of redshifts of this particular spectral line.

380,000 and 65 million years ABB correspond to a cosmological redshift (z) of 1,081 and 40, respectively. We can calculate what the observed wavelength and frequency of the 21 cm line would be for the beginning and end of the dark ages.

\lambda _{obs} = (z+1)\cdot \lambda_{emit}


The observed wavelength (λobs) for the 21 cm line (λemit) at redshift (z) of 1,081 using the above equation gives us 22,836.8 cm or 228.4 meters.

\nu = \frac{c}{\lambda }


That gives us a frequency (ν) of 1.3 MHz (using the equation above), where the speed of light c = 299,792,458 meters per second.

So a 21 cm line emitted 380,000 years ABB will be observed to have a wavelength of 228.4 m and a frequency of 1.3 MHz.

Using the same equations, we find that a 21 cm line emitted 65 Myr ABB will be observed to have a wavelength of 8.7 m and a frequency of 34.7 MHz.

We thus will be quite interested in taking a detailed look at radio waves in the entire frequency range 1.3 – 34.7 MHz, with corresponding wavelengths from 228.4 m down to 8.7 m.2

The interference from the Earth’s ionosphere and the ever-increasing cacophony of humanity’s radio transmissions makes observing these faint radio signals all but impossible from anywhere on or near the Earth. Radio astronomers and observational cosmologists are planning to locate radio telescopes on the far side of the Moon—both on the surface and in orbit above it—where the entire mass of the Moon will effectively block all terrestrial radio interference. There we will finally hear the radio whispers of matter before the first stars formed.

1 By “neutral” we mean hydrogen atoms where the electron has not been ionized and resides in the ground state—not an excited state.

2 Incidentally, the 2.7 K cosmic microwave background radiation which is the “afterglow” of the Big Bang itself at the beginning of the dark ages (380,000 years ABB), peaks at a frequency between 160 and 280 GHz and a wavelength around 1 – 2 mm. So this is a much higher frequency and shorter wavelength than the redshifted 21 cm emissions we are proposing to observe here.

References

Ananthaswamy, Anil, “The View from the Far Side of the Moon”, Scientific American, April 2021, pp. 60-63

Burns, Jack O., et al., “Global 21-cm Cosmology from the Farside of the Moon”, https://arxiv.org/ftp/arxiv/papers/2103/2103.05085.pdf

Koopmans, Léon, et al., “Peering into the Dark (Ages) with Low-Frequency Space Interferometers”, https://arxiv.org/ftp/arxiv/papers/1908/1908.04296.pdf

Ned Wright’s Javascript Cosmology Calculator, http://www.astro.ucla.edu/~wright/CosmoCalc.html

Video Meteors 2020 – II

During the second half of 2020, I serendipitously captured six meteors on my telescope’s 17 x 11 arcminute video field of view while observing potential asteroid occultation events. I used the method described in There’s a Meteor in My Image to determine the radiant for each meteor. Here they are.

Sporadic meteor 8 Jul 2020 UT; Field location TYC 7379-00569-1 in Scorpius
Each frame is an exposure of 0.13s (meteor is at left side of field)

A sporadic meteor is any meteor that does not come from a known radiant.

Probable sporadic meteor 22 Aug 2020 UT; Field location UCAC4 394-071682 in Serpens
Each frame is an exposure of 0.13s ; possibly a Perseid (meteor from upper right to lower left)
Probable sporadic meteor 29 Aug 2020 UT; Field location UCAC4 601-019523 in Auriga
Each frame is an exposure of 0.27s ; possibly a Perseid (meteor from upper right to lower left)
Orionid 11 Oct 2020 UT; Field location TYC 1337-01489-1 in Gemini; very fast!
Each frame is an exposure of 0.13s ; (meteor at upper right)
Sporadic meteor 14 Nov 2020 UT; Field location UCAC4 559-043312 in Gemini
Each frame is an exposure of 0.27s (meteor at upper right)
Probable Leonid 5 Dec 2020 UT; Field location UCAC4 410-001419 in Cetus
Each frame is an exposure of 0.13s ; (meteor along upper part of field)

None of these meteors were particularly bright, unfortunately, so you may want to use the full-screen button at the lower-right-hand corner of each video to see them well.

References

International Meteor Organization, 2o2o Meteor Shower Calendar, Jürgen Rendtel, ed. https://www.imo.net/files/meteor-shower/cal2020.pdf.

Supernovae in the Milky Way

The first recorded supernova in our Milky Way galaxy (or anywhere else, for that matter) was seen to blaze forth in the constellation Centaurus by astute Chinese astronomers in 185 AD. Including that one, only seven confirmed supernovae have been observed in our Milky Way galaxy, though thousands are discovered each year in other galaxies.

Supernova light reached Earth in AD 185, 393, 1006, 1054, 1181, 1572, and 1604. All seven of these events occurred before the invention of the telescope. Are we overdue for another supernova? Well, given this ridiculously small sample, we can endeavor to do some simple “statistics”. The shortest recorded interval between two Milky Way supernovae was 32 years between 1572 and 1604. The longest interval has been 613 years, between the supernovae of 393 and 1006 (assuming none went unnoticed). On average then (such as it is), we “should” have seen a Milky Way supernova around 1841, and using the longest interval of 613 years, we might be expecting one by the year 2217. Undoubtedly, some supernovae in the Milky Way have escaped detection because they lay behind thick interstellar clouds.

The big mystery to me is why are there no recorded supernova events prior to 185 AD? The earliest extant records of astronomical events go back at least as far as 2316 BC (a comet in the constellation Crater was recorded by Chinese astronomers), but in the intervening 2,500 years there has been no mention of anything that could be attributed to a supernova. Or has there? Some writings before and after 185 AD suggest possible supernovae, but until a supernova remnant is identified, we need to look for other explanations.

Here follows a table of the known observed Milky Way supernovae. Of course, other supernova remnants have been discovered in our Milky Way galaxy, but no record has yet been discovered describing these events. Many of them predate recorded history.

In the table below, you’ll note that these supernovae tend to lie close to the galactic plane (galactic latitude b = 0°)—not at all surprising considering that’s where most of the stars are.

Milky Way Supernovae confirmed to have been observed

Zodiacal Light 2021

In 2021, the best dates and times for observing the zodiacal light are listed in the calendar below. The sky must be very clear with little or no light pollution. The specific times listed are for Dodgeville, Wisconsin (42° 58′ N, 90° 08′ W).

Here’s a nicely-formatted printable PDF file of the zodiacal light calendar:

January 2021
SUN MON TUE WED THU FRI SAT
          1 2
3 4 5 6 7 8 9
10 11 12 13 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30
Zodiacal Light 6:49 – 7:26 p.m. West
31
Zodiacal Light 6:50 – 7:50 p.m. West
           

February 2021
SUN MON TUE WED THU FRI SAT
  1
Zodiacal Light 6:51 – 7:51 p.m. West
2
Zodiacal Light 6:52 – 7:52 p.m. West
3
Zodiacal Light 6:53 – 7:53 p.m. West
4
Zodiacal Light 6:54 – 7:54 p.m. West
5
Zodiacal Light 6:56 – 7:56 p.m. West
6
Zodiacal Light 6:57 – 7:57 p.m. West
7
Zodiacal Light 6:58 – 7:58 p.m. West
8
Zodiacal Light 6:59 – 7:59 p.m. West
9
Zodiacal Light 7:00 – 8:00 p.m. West
10
Zodiacal Light 7:02 – 8:02 p.m. West
11
Zodiacal Light 7:03 – 8:03 p.m. West
12
Zodiacal Light 7:04 – 8:04 p.m. West
13
14 15 16 17 18 19 20
21 22 23 24 25 26 27
28
Zodiacal Light 7:23 – 7:36 p.m. West
           

March 2021
SUN MON TUE WED THU FRI SAT
  1
Zodiacal Light 7:25 – 8:25 p.m. West
2
Zodiacal Light 7:26 – 8:26 p.m. West
3
Zodiacal Light 7:27 – 8:27 p.m. West
4
Zodiacal Light 7:28 – 8:28 p.m. West
5
Zodiacal Light 7:29 – 8:29 p.m. West
6
Zodiacal Light 7:31 – 8:31 p.m. West
7
Zodiacal Light 7:32 – 8:32 p.m. West
8
Zodiacal Light 7:33 – 8:33 p.m. West
9
Zodiacal Light 7:34 – 8:34 p.m. West
10
Zodiacal Light 7:36 – 8:36 p.m. West
11
Zodiacal Light 7:37 – 8:37 p.m. West
12
Zodiacal Light 7:38 – 8:38 p.m. West
13
Zodiacal Light 7:40 – 8:40 p.m. West
14
Zodiacal Light 8:41 – 9:41 p.m. West
15 16 17 18 19 20
21 22 23 24 25 26 27
28 29 30
Zodiacal Light 9:03 – 10:03 p.m. West
31
Zodiacal Light 9:04 – 10:04 p.m. West
     

April 2021
SUN MON TUE WED THU FRI SAT
        1
Zodiacal Light 9:05 – 10:05 p.m. West
2
Zodiacal Light 9:07 – 10:07 p.m. West
3
Zodiacal Light 9:08 – 10:08 p.m. West
4
Zodiacal Light 9:10 – 10:10 p.m. West
5
Zodiacal Light 9:11 – 10:11 p.m. West
6
Zodiacal Light 9:13 – 10:13 p.m. West
7
Zodiacal Light 9:14 – 10:14 p.m. West
8
Zodiacal Light 9:16 – 10:16 p.m. West
9
Zodiacal Light 9:17 – 10:17 p.m. West
10
Zodiacal Light 9:19 – 10:19 p.m. West
11
Zodiacal Light 9:20 – 10:20 p.m. West
12
Zodiacal Light 9:22 – 10:22 p.m. West
13
Zodiacal Light 9:24 – 10:24 p.m. West
14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30  

May 2021
SUN MON TUE WED THU FRI SAT
            1
2 3 4 5 6 7 8
9 10 11 12 13 14 15
16 17 18 19 20 21 22
23 24 25 26 27 28 29
30 31          

June 2021
SUN MON TUE WED THU FRI SAT
    1 2 3 4 5
6 7 8 9 10 11 12
13 14 15 16 17 18 19
20 21 22 23 24 25 26
27 28 29 30      

July 2021
SUN MON TUE WED THU FRI SAT
        1 2 3
4 5 6 7 8 9 10
11 12 13 14 15 16 17
18 19 20 21 22 23 24
25 26 27 28 29 30 31

August 2021
SUN MON TUE WED THU FRI SAT
1 2 3 4 5 6 7
8 9 10 11 12 13 14
15 16 17 18 19 20 21
22 23 24 25 26 27 28
29 30 31        

September 2021
SUN MON TUE WED THU FRI SAT
      1 2 3 4
5 6
Zodiacal Light 3:52 – 4:52 a.m. East
7
Zodiacal Light 3:53 – 4:53 a.m. East
8
Zodiacal Light 3:54 – 4:54 a.m. East
9
Zodiacal Light 3:56 – 4:56 a.m. East
10
Zodiacal Light 3:57 – 4:57 a.m. East
11
Zodiacal Light 3:58 – 4:58 a.m. East
12
Zodiacal Light 4:00 – 5:00 a.m. East
13
Zodiacal Light 4:01 – 5:01 a.m. East
14
Zodiacal Light 4:02 – 5:02 a.m. East
15
Zodiacal Light 4:04 – 5:04 a.m. East
16
Zodiacal Light 4:05 – 5:05 a.m. East
17
Zodiacal Light 4:06 – 5:06 a.m. East
18
Zodiacal Light 4:08 – 5:08 a.m. East
19
Zodiacal Light 4:59 – 5:09 a.m. East
20 21 22 23 24 25
26 27 28 29 30    

October 2021
SUN MON TUE WED THU FRI SAT
          1 2
3 4 5
Zodiacal Light 4:28 – 5:28 a.m. East
6
Zodiacal Light 4:30 – 5:30 a.m. East
7
Zodiacal Light 4:31 – 5:31 a.m. East
8
Zodiacal Light 4:32 – 5:32 a.m. East
9
Zodiacal Light 4:33 – 5:33 a.m. East
10
Zodiacal Light 4:34 – 5:34 a.m. East
11
Zodiacal Light 4:35 – 5:35 a.m. East
12
Zodiacal Light 4:37 – 5:37 a.m. East
13
Zodiacal Light 4:38 – 5:38 a.m. East
14
Zodiacal Light 4:39 – 5:39 a.m. East
15
Zodiacal Light 4:40 – 5:40 a.m. East
16
Zodiacal Light 4:41 – 5:41 a.m. East
17
Zodiacal Light 4:42 – 5:42 a.m. East
18
Zodiacal Light 5:03 – 5:43 a.m. East
19 20 21 22 23
24 25 26 27 28 29 30
31            

November 2021
SUN MON TUE WED THU FRI SAT
  1 2 3 4
Zodiacal Light 5:03 – 6:03 a.m. East
5
Zodiacal Light 5:04 – 6:04 a.m. East
6
Zodiacal Light 5:05 – 6:05 a.m. East
7
Zodiacal Light 4:06 – 5:06 a.m. East
8
Zodiacal Light 4:07 – 5:07 a.m. East
9
Zodiacal Light 4:08 – 5:08 a.m. East
10
Zodiacal Light 4:09 – 5:09 a.m. East
11
Zodiacal Light 4:10 – 5:10 a.m. East
12
Zodiacal Light 4:12 – 5:12 a.m. East
13
Zodiacal Light 4:13 – 5:13 a.m. East
14
Zodiacal Light 4:14 – 5:14 a.m. East
15
Zodiacal Light 4:15 – 5:15 a.m. East
16
Zodiacal Light 4:16 – 5:16 a.m. East
17
Zodiacal Light 5:06 – 5:17 a.m. East
18 19 20
21 22 23 24 25 26 27
28 29 30        

December 2021
SUN MON TUE WED THU FRI SAT
      1 2 3 4
5 6 7 8 9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29 30 31  

The best nights to observe the zodiacal light at mid-northern latitudes occur when the ecliptic plane intersects the horizon at an angle of 60° or steeper. The dates above were chosen on that basis, with the Sun at least 18° below the horizon and the Moon below the horizon being used to calculate the times. An interval of time of one hour either before morning twilight or after evening twilight was chosen arbitrarily because it is the “best one hour” for observing the zodiacal light. The zodiacal light cone will be brightest and will reach highest above the horizon when the Sun is 18° below the horizon (astronomical twilight), but no less.

If you are interested in calculating the angle the ecliptic makes with your horizon for any date and time, you can use the following formula:

\cos I = \cos \varepsilon \sin \phi-\sin \varepsilon \cos \phi \sin \theta

where I is the angle between the ecliptic and the horizon, ε is  the obliquity of the ecliptic, φ is the latitude of the observer, and θ is the local sidereal time (the right ascension of objects on the observer's meridian at the time of observation).

Here’s a SAS program I wrote to do these calculations:

References
Meeus, J. Astronomical Algorithms. 2nd ed., Willmann-Bell, 1998, p. 99.

Meteor Shower Calendar 2021

Here’s our meteor shower calendar for 2021.  It is sourced from the IMO’s Working List of Visual Meteor Showers (https://www.imo.net/files/meteor-shower/cal2021.pdf, Table 5, p. 25).

Each meteor shower is identified using its three-character IAU meteor shower code.  Codes are bold on the date of maximum, and one day either side of maximum.

Here’s a printable PDF file of the meteor shower calendar shown below:

Happy meteor watching!

January 2021
SUN MON TUE WED THU FRI SAT
          1
DLM QUA
2
DLM QUA
3
DLM QUA
4
DLM QUA
5
DLM QUA
6
DLM QUA
7
DLM QUA
8
DLM QUA
9
DLM QUA
10
DLM QUA GUM
11
DLM QUA GUM
12
DLM QUA GUM
13
DLM GUM
14
DLM GUM
15
DLM GUM
16
DLM GUM
17
DLM GUM
18
DLM GUM
19
DLM GUM
20
DLM GUM
21
DLM GUM
22
DLM GUM
23
DLM
24
DLM
25
DLM
26
DLM
27
DLM
28
DLM
29
DLM
30
DLM
31
DLM ACE
           
February 2021
SUN MON TUE WED THU FRI SAT
  1
DLM ACE
2
DLM ACE
3
DLM ACE
4
DLM ACE
5
ACE
6
ACE
7
ACE
8
ACE
9
ACE
10
ACE
11
ACE
12
ACE
13
ACE
14
ACE
15
ACE
16
ACE
17
ACE
18
ACE
19
ACE
20
ACE
21 22 23 24 25
GNO
26
GNO
27
GNO
28
GNO
           
March 2021
SUN MON TUE WED THU FRI SAT
  1
GNO
2
GNO
3
GNO
4
GNO
5
GNO
6
GNO
7
GNO
8
GNO
9
GNO
10
GNO
11
GNO
12
GNO
13
GNO
14
GNO
15
GNO
16
GNO
17
GNO
18
GNO
19
GNO
20
GNO
21
GNO
22
GNO
23
GNO
24
GNO
25
GNO
26
GNO
27
GNO
28
GNO
29 30 31      
April 2021
SUN MON TUE WED THU FRI SAT
        1 2 3
4 5 6 7 8 9 10
11 12 13 14
LYR
15
PPU LYR
16
PPU LYR
17
PPU LYR
18
PPU LYR
19
ETA PPU LYR
20
ETA PPU LYR
21
ETA PPU LYR
22
ETA PPU LYR
23
ETA PPU LYR
24
ETA PPU LYR
25
ETA PPU LYR
26
ETA PPU LYR
27
ETA PPU LYR
28
ETA PPU LYR
29
ETA LYR
30
ETA LYR
 
May 2021
SUN MON TUE WED THU FRI SAT
            1
ETA
2
ETA
3
ELY ETA
4
ELY ETA
5
ELY ETA
6
ELY ETA
7
ELY ETA
8
ELY ETA
9
ELY ETA
10
ELY ETA
11
ELY ETA
12
ELY ETA
13
ELY ETA
14
ARI ELY ETA
15
ARI ETA
16
ARI ETA
17
ARI ETA
18
ARI ETA
19
ARI ETA
20
ARI ETA
21
ARI ETA
22
ARI ETA
23
ARI ETA
24
ARI ETA
25
ARI ETA
26
ARI ETA
27
ARI ETA
28
ARI ETA
29
ARI
30
ARI
31
ARI
         
June 2021
SUN MON TUE WED THU FRI SAT
    1
ARI
2
ARI
3
ARI
4
ARI
5
ARI
6
ARI
7
ARI
8
ARI
9
ARI
10
ARI
11
ARI
12
ARI
13
ARI
14
ARI
15
ARI
16
ARI
17
ARI
18
ARI
19
ARI
20
ARI
21
ARI
22
JBO ARI
23
JBO ARI
24
JBO ARI
25
JBO
26
JBO
27
JBO
28
JBO
29
JBO
30
JBO
     
July 2021
SUN MON TUE WED THU FRI SAT
        1
JBO
2
JBO
3
CAP
4
CAP
5
CAP
6
CAP
7
CAP
8
CAP
9
CAP
10
CAP
11
CAP
12
CAP SDA
13
CAP SDA
14
CAP SDA
15
CAP SDA PAU
16
CAP SDA PAU
17
PER CAP SDA PAU
18
PER CAP SDA PAU
19
PER CAP SDA PAU
20
PER CAP SDA PAU
21
PER CAP SDA PAU
22
PER CAP SDA PAU
23
PER CAP SDA PAU
24
PER CAP SDA PAU
25
PER CAP SDA PAU
26
PER CAP SDA PAU
27
PER CAP SDA PAU
28
PER CAP SDA PAU
29
PER CAP SDA PAU
30
PER CAP SDA PAU
31
PER CAP SDA PAU
August 2021
SUN MON TUE WED THU FRI SAT
1
PER CAP SDA PAU
2
PER CAP SDA PAU
3
KCG PER CAP SDA PAU
4
KCG PER CAP SDA PAU
5
KCG PER CAP SDA PAU
6
KCG PER CAP SDA PAU
7
KCG PER CAP SDA PAU
8
KCG PER CAP SDA PAU
9
KCG PER CAP SDA PAU
10
KCG PER CAP SDA PAU
11
KCG PER CAP SDA
12
KCG PER CAP SDA
13
KCG PER CAP SDA
14
KCG PER CAP SDA
15
KCG PER CAP SDA
16
KCG PER SDA
17
KCG PER SDA
18
KCG PER SDA
19
KCG PER SDA
20
KCG PER SDA
21
KCG PER SDA
22
KCG PER SDA
23
KCG PER SDA
24
KCG PER
25
KCG
26 27 28
AUR
29
AUR
30
AUR
31
AUR
       
September 2021
SUN MON TUE WED THU FRI SAT
      1
AUR
2
AUR
3
AUR
4
AUR
5
SPE AUR
6
SPE
7
SPE
8
SPE
9
DSX SPE
10
STA DSX SPE
11
STA DSX SPE
12
STA DSX SPE
13
STA DSX SPE
14
STA DSX SPE
15
STA DSX SPE
16
STA DSX SPE
17
STA DSX SPE
18
STA DSX SPE
19
STA DSX SPE
20
STA DSX SPE
21
STA DSX SPE
22
STA DSX
23
STA DSX
24
STA DSX
25
STA DSX
26
STA DSX
27
STA DSX
28
STA DSX
29
STA DSX
30
STA DSX
   
October 2021
SUN MON TUE WED THU FRI SAT
          1
STA DSX
2
ORI STA DSX
3
ORI STA DSX
4
ORI STA OCT DSX
5
ORI STA OCT DSX
6
ORI STA DRA OCT DSX
7
ORI STA DRA DSX
8
ORI STA DRA DSX
9
ORI STA DRA DSX
10
ORI DAU STA DRA
11
ORI DAU STA
12
ORI DAU STA
13
ORI DAU STA
14
ORI EGE DAU STA
15
ORI EGE DAU STA
16
ORI EGE DAU STA
17
ORI EGE DAU STA
18
ORI EGE DAU STA
19
LMI ORI EGE STA
20
NTA LMI ORI EGE STA
21
NTA LMI ORI EGE STA
22
NTA LMI ORI EGE STA
23
NTA LMI ORI EGE STA
24
NTA LMI ORI EGE STA
25
NTA LMI ORI EGE STA
26
NTA LMI ORI EGE STA
27
NTA LMI ORI EGE STA
28
NTA ORI STA
29
NTA ORI STA
30
NTA ORI STA
31
NTA ORI STA
           
November 2021
SUN MON TUE WED THU FRI SAT
  1
NTA ORI STA
2
NTA ORI STA
3
NTA ORI STA
4
NTA ORI STA
5
NTA ORI STA
6
LEO NTA ORI STA
7
LEO NTA ORI STA
8
LEO NTA STA
9
LEO NTA STA
10
LEO NTA STA
11
LEO NTA STA
12
LEO NTA STA
13
NOO LEO NTA STA
14
NOO LEO NTA STA
15
NOO AMO LEO NTA STA
16
NOO AMO LEO NTA STA
17
NOO AMO LEO NTA STA
18
NOO AMO LEO NTA STA
19
NOO AMO LEO NTA STA
20
NOO AMO LEO NTA STA
21
NOO AMO LEO NTA
22
NOO AMO LEO NTA
23
NOO AMO LEO NTA
24
NOO AMO LEO NTA
25
NOO AMO LEO NTA
26
NOO LEO NTA
27
NOO LEO NTA
28
PHO NOO LEO NTA
29
PHO NOO LEO NTA
30
PHO NOO LEO NTA
       
December 2021
SUN MON TUE WED THU FRI SAT
      1
PUP PHO NOO NTA
2
PUP PHO NOO NTA
3
HYD PUP PHO NOO NTA
4
GEM HYD PUP PHO NOO NTA
5
DLM GEM HYD MON PUP PHO NOO NTA
6
DLM GEM HYD MON PUP PHO NOO NTA
7
DLM GEM HYD MON PUP PHO NTA
8
DLM GEM HYD MON PUP PHO NTA
9
DLM GEM HYD MON PUP PHO NTA
10
DLM GEM HYD MON PUP NTA
11
DLM GEM HYD MON PUP
12
DLM COM GEM HYD MON PUP
13
DLM COM GEM HYD MON PUP
14
DLM COM GEM HYD MON PUP
15
DLM COM GEM HYD MON PUP
16
DLM COM GEM HYD MON
17
DLM URS COM GEM HYD MON
18
DLM URS COM GEM HYD MON
19
DLM URS COM GEM HYD MON
20
DLM URS COM GEM HYD MON
21
DLM URS COM
22
DLM URS COM
23
DLM URS COM
24
DLM URS
25
DLM URS
26
DLM URS
27
DLM
28
DLM QUA
29
DLM QUA
30
DLM QUA
31
DLM QUA
 

Earliest Sunset, Latest Sunrise

Why does the Earliest Sunset come before the Winter Solstice and the Latest Sunrise after?


Why does the Earliest Sunrise come before the Summer Solstice and the Latest Sunset after?

Ever wonder? I have. And aside from some hand-wavy explanations, I’ve never been able to explain this very well. Here’s the best explanation I have seen yet, provided in the December 2007 issue of Sky & Telescope, p. 55:

You’d think the earliest sunset would come on the shortest day (or longest night) of the year, at the winter solstice. But in fact, the day-night cycle shifts back and forth a little with the seasons, due to the tilt of Earth’s axis and the ellipticity of Earth’s orbit. At the beginning of December, sunrise, midday, and sunset all happen a little earlier than they “should”, and in January they run a little late. So the earliest sunset ends up being two or three weeks before the solstice, and the latest sunrise is two or three weeks afterward. The exact dates depend on your latitude.

Continuing along that same line of thought…

At the beginning of June, sunrise, midday, and sunset all happen a little later than they “should” and in July they run a little earlier. So the earliest sunrise ends up being about a week before the solstice, and the latest sunset is about a week afterwards. The exact dates depend on your latitude.

I know, I know. You still have a question. “Why are the dates of earliest sunrise and latest sunset closer to the summer solstice than the dates of earliest sunset and latest sunrise to the winter solstice?” Good question. I think it has everything to do with the fact that the Earth is near aphelion at the time of the summer solstice, and thus moving most slowly in its orbit around the Sun (the Earth’s orbit is slightly elliptical and not circular). That means that the Sun is moving slowest against the background stars and thus the accumulated difference between the sidereal day and solar day is the smallest at that time of year. That means the spread of days between earliest sunrise and latest sunset is less. Conversely, at the winter solstice, Earth is near perihelion, and therefore it is moving most quickly in its orbit around the Sun. That means that the Sun is moving fastest against the background stars and thus the accumulated difference between the sidereal day and solar day is largest at that time of year. That means the spread of days between earliest sunset and latest sunrise is more.

Here in Dodgeville, Wisconsin, where the latitude is just shy of 43˚ N and the longitude is just a tad over 90˚ W, the earliest sunset this year is today, Tuesday, December 8, 2020, at 4:25:49 p.m.

Latest sunrise in 2021 will be on both Saturday, January 2 and Sunday, January 3 at 7:31:51 a.m.

Pause to consider that if we were on year-round daylight saving time, latest sunrise wouldn’t be until 8:31:51 a.m.

My preference would be to stay on standard time year-round, as Arizona does.

Why Did It Take a Telescope to Discover the Orion Nebula?

Using the newly-invented telescope, French astronomer Nicolas-Claude Fabri de Peiresc (1580-1637) discovered the now-famous Orion Nebula (M42) when he was 29 years old, 410 years ago on this day.

November 26, 1610.

But wait a minute. You and I can see a nebulous “star” below the belt of Orion with our unaided eyes under a reasonably dark sky. Why wasn’t this object discovered long before the invention of the telescope?

Apparently, there is no known report of a “nebulous star” in the sword of Orion prior to Peiresc’s discovery. Is the Orion nebula brighter now than it was a few centuries ago? Is it possible an earlier observational report somehow got missed or was not properly interpreted?

There is speculation that the Maya civilization of Mesoamerica recognized the Orion Nebula long before Peiresc’s discovery, describing it as smoke from the smoldering embers of creation.

One can only stand in wonderment at the knowledge and experiences of hundreds of generations of men, women, and children who are utterly unknown to us today. Passed from person to person and generation to generation through oral tradition, never written down and eventually lost. Or written down on documents that later disintegrated or were purposefully destroyed.

Who hasn’t wished that they could could time travel back to the past? Have you ever wondered what your current location looked like a hundred years ago? A thousand years ago? Ten thousand or more years ago? Though sending humans into the past will probably never be possible, who’s to say that we won’t eventually figure out a way to view and perhaps even hear the past, without actually being there or having the ability to change it?