10/06/2021 – Ephemeris – Let’s search for the naked-eye planets for this week

October 6, 2021 Comments off

This is Ephemeris for Wednesday, October 6th. Today the Sun will be up for 11 hours and 26 minutes, setting at 7:13, and it will rise tomorrow at 7:48. The Moon is new today, and won’t be visible.

Let’s search for the naked-eye planets for this week. Venus should be visible in the southwestern evening twilight by 7:45 tonight. It will set at 8:52 pm. By 8 pm, Jupiter will be spotted in the south-southeastern sky. The Jupiter should be easy to spot at that hour. Saturn will be dimmer, but a bit higher and to its right. They will be visible into the morning hours, with Saturn setting first at 1:58 am, and Jupiter following at 3:20. Saturn’s rings can be seen in a spotting scope of about 20 power magnification. Though at that power the rings won’t appear separated from the planet, so Saturn will look like an elliptical disk. Jupiter’s 4 brightest moons are spread out, three on one side, and one on the other. They might all be visible in binoculars tonight.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.

Addendum

Venus in evening twilight

Venus in evening twilight at 7:45 pm, about a half hour after sunset. Created using Stellarium.

Jupiter and Saturn at 8 pm

Jupiter and Saturn at 8 pm, about 45 minutes after sunset tonight, October 6, 2021. Created using Stellarium.

Telescopic views of the naked-eye planets

Telescopic views of the bright planets (north up) as they would be seen in a small telescope, with the same magnification, this evening at 8 pm, October 6, 2021. Apparent diameters: Venus, 19.81″, 59.8% illuminated; Saturn 17.48″, its rings 40.73″; Jupiter, 45.55″. The ” symbol means seconds of arc (1/3600th of a degree.) Click on the image to enlarge it. Created using Cartes du Ciel (Sky Charts).

Planets and the Moon overnight tonight

The naked-eye planets and the Moon at sunset and sunrise on a single night, starting with sunset on the right on October 6, 2021. The night ends on the left with sunrise on the 7th. Click on the image to enlarge it. Created using my LookingUp program.

10/05/2021 – Ephemeris – Can you spot the North American Nebula?

October 5, 2021 Comments off

This is Bob Moler with Ephemeris for Tuesday, October 5th. Today the Sun will be up for 11 hours and 29 minutes, setting at 7:15, and it will rise tomorrow at 7:47. The Moon, 1 day before new, will rise at 7:46 tomorrow morning.

Most of what we see in the Milky Way are just masses of stars, but there are bright clouds of gas, or to name them properly: emission nebulae. These bright clouds are areas of star formation. It is the ultraviolet light from young massive stars that light up the clouds they were formed from. A bright one, easily visible in binoculars, is just about overhead at 9 p.m. Called the North American Nebula, a glow, that in photographs is shaped much like our continent, is just east of the star Deneb which is practically overhead in the evening. Deneb is the northernmost star of the Summer Triangle, and brightest star in Cygnus the swan or Northern Cross. There are many other nebulae in the Milky Way, visible in binoculars and small telescopes. Many enjoyable hours can be spent sweeping the Milky Way for nebulae and star clusters.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.

Addendum

North American Nebula finder animation

North American Nebula finder animation. I’ve dimmed down the stars a bit and increased the brightness of the Milky Way to aid in spotting the nebula. It requires dark skies to see it. I believe I can make it out with the naked eye too. Click on the image to enlarge it. Created using Stellarium and GIMP.

Deneb & North American Nebula

One of my old photographs of Deneb and the North American Nebula, digitized from a slide.

Better view of the North American Nebula taken by Scott Anttila.

Better view of the North American Nebula taken by Scott Anttila.

10/04/2021 – Ephemeris – Why we can’t talk to the Perseverance rover on Mars right now

October 4, 2021 Comments off

This is Ephemeris for Monday, October 4th. Today the Sun will be up for 11 hours and 32 minutes, setting at 7:17, and it will rise tomorrow at 7:46. The Moon, 2 days before new, will rise at 6:30 tomorrow morning.

NASA is no longer sending commands to its Perseverance rover or any of its assets roving or orbiting Mars now. The reason isn’t particularly sinister. It’s the approximately 26 month Mars solar conjunction. The Sun is a noisy radio source, and commands sent to or data received from these martian assets could be garbled. This affects everyone’s assets on or orbiting Mars, which includes the Europeans, India, China and the United Arab Emirates. For NASA, communication restrictions started two days ago and will last until the 14th. This will give the folks at JPL who are operating the Curiosity and Perseverance rovers some time off, and time to plan the next few months of activity.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.

Addendum

Mars in solar conjunction.

Mars in solar conjunction. Looking at the inner solar system.  Mars, near the bottom of the image, is 244.6 million miles (393.9 million kilometers) from Earth. Click on the image to enlarge it. Credit NASA’s Eyes app.

Mars beyond and to the upper left of the Sun yesterday

Mars beyond and to the upper left of the Sun yesterday. It’s tough to get intelligible radio signals through the solar corona. Credit: NASA/ESA SOHO* spacecraft. The annotation is mine.

* SOlar and Heliospheric Observatory, spacecraft in halo orbit around the Lagrangian L1 equilibrium point about  930,000 miles (1,500,000 kilometers) sunward of the Earth. This keeps the satellite roughly between the Sun and the Earth, instead of moving ahead of the Earth because it’s closer to the Sun.

10/01/2021 Ephemeris – Learn about exoplanets and have a star party tonight (virtually)

October 1, 2021 Comments off

This is Bob Moler with Ephemeris for Friday, October 1st. Today the Sun will be up for 11 hours and 41 minutes, setting at 7:22, and it will rise tomorrow at 7:42. The Moon, 3 days past last quarter, will rise at 2:49 tomorrow morning.

The Grand Traverse Astronomical Society hold a virtual meeting and program at 8 pm tonight via Zoom. For the program, yours truly will present Finding Exoplanets. Exoplanets are planets found orbiting other stars or rogue planets floating in interstellar space alone. We’ve actually only seen a handful of exoplanets, so most are detected by their effects on their host star. A virtual star party will start around 9 pm, hosted by Dr. Jerry Dobek, astronomy professor at Northwestern Michigan College, but only if it’s clear in Traverse City. It will feature Jupiter and Saturn, wonders of the northern part of the summer Milky Way. Instructions to join the meeting and a link can be found on the society’s website, http://www.gtastro.org.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.

Addendum

Finding Exoplanets title slide

Finding Exoplanets title slide.

09/30/2021 – Ephemeris – Previewing October skies

September 30, 2021 Comments off

This is Ephemeris for Thursday, September 30th. Today the Sun will be up for 11 hours and 44 minutes, setting at 7:24, and it will rise tomorrow at 7:41. The Moon, 2 days past last quarter, will rise at 1:42 tomorrow morning.

Let’s look at the skies for this month of October. The Sun will still be moving south rapidly. Daylight hours in the Interlochen/Traverse City area will drop from 11 hours and 41 minutes tomorrow to 10 hours 12 minutes on the 31st. The altitude of the sun above the southern horizon at local noon will be 42 degrees tomorrow, and will descend to 31 degrees on Halloween, also in the Interlochen/Traverse City area. The Straits area will have the sun a degree lower. Local noon, when the Sun is due south, will be about 1:30 pm in Interlochen and Traverse City. Bits of Halley’s comet will return later in the month as the Orionid meteor shower. However, it will have to compete with a nearly full moon.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.

October Evening Star Chart

Star Chart October

Star Chart for October 2021. (9 p.m. EDT, October 15, 2021) Click on image to enlarge. Created using my LookingUp program. Click on image to enlarge.

The planets and stars are plotted for the 15th at 9 p.m. EDT in the evening and 6 a.m. for the morning chart. These are the chart times. Note that Traverse City is located approximately 45 minutes behind our time meridian, West 75° longitude. (An hour 45 minutes behind our daylight saving time meridian during EDT). To duplicate the star positions on a planisphere you may have to set it to 1 hour 45 minutes earlier than the current time.

October Morning Star Chart

Star Chart October Mornings

Star Chart for October mornings, 2021 (6 a.m. EDT, October 15, 2021). Click on image to enlarge. Created using my LookingUp program.

For a list of constellation names to go with the abbreviations, click here.

  • Pointer stars at the front of the bowl of the Big Dipper point to Polaris, the North Star.
  • Leaky dipper drips on Leo.
  • Follow the arc of the handle of the Big Dipper to the star Arcturus.
  • The Summer Triangle is in red.
  • OriR – Orionid Radiant – Peak 22nd – Zenithal Hourly Rate = 20

Twilight Limits, Nautical and Astronomical

EDT
Morning Twilight Evening Twilight Dark Night Moon
Date Astro. Nautical Nautical Astro. Start End Illum.
2021-10-01 6h09m 6h43m 20h29m 21h03m 21h03m 1h42m 0.21
2021-10-02 6h10m 6h44m 20h27m 21h01m 21h01m 2h49m 0.13
2021-10-03 6h11m 6h45m 20h25m 20h59m 20h59m 4h01m 0.07
2021-10-04 6h13m 6h47m 20h23m 20h57m 20h57m 5h15m 0.02
2021-10-05 6h14m 6h48m 20h21m 20h55m 20h55m 6h14m 0
2021-10-06 6h15m 6h49m 20h19m 20h54m 20h54m 6h15m 0.01
2021-10-07 6h16m 6h50m 20h18m 20h52m 20h52m 6h16m 0.04
2021-10-08 6h18m 6h52m 20h16m 20h50m 20h50m 6h18m 0.1
2021-10-09 6h19m 6h53m 20h14m 20h48m 21h10m 6h19m 0.19
2021-10-10 6h20m 6h54m 20h12m 20h46m 21h53m 6h20m 0.29
2021-10-11 6h21m 6h55m 20h11m 20h44m 22h46m 6h21m 0.4
2021-10-12 6h23m 6h57m 20h09m 20h43m 23h50m 6h23m 0.51
2021-10-13 6h24m 6h58m 20h07m 20h41m 6h24m 0.62
2021-10-14 6h25m 6h59m 20h06m 20h39m 1h00m 6h25m 0.72
2021-10-15 6h26m 7h00m 20h04m 20h38m 2h13m 6h26m 0.81
2021-10-16 6h28m 7h01m 20h02m 20h36m 3h25m 6h28m 0.89
2021-10-17 6h29m 7h03m 20h01m 20h34m 4h35m 6h29m 0.94
2021-10-18 6h30m 7h04m 19h59m 20h33m 5h43m 6h30m 0.98
2021-10-19 6h31m 7h05m 19h57m 20h31m 1
2021-10-20 6h33m 7h06m 19h56m 20h30m 1
2021-10-21 6h34m 7h08m 19h54m 20h28m 0.98
2021-10-22 6h35m 7h09m 19h53m 20h26m 0.94
2021-10-23 6h36m 7h10m 19h51m 20h25m 0.89
2021-10-24 6h37m 7h11m 19h50m 20h24m 20h24m 20h56m 0.82
2021-10-25 6h39m 7h12m 19h48m 20h22m 20h22m 21h38m 0.74
2021-10-26 6h40m 7h14m 19h47m 20h21m 20h21m 22h29m 0.66
2021-10-27 6h41m 7h15m 19h45m 20h19m 20h19m 23h27m 0.56
2021-10-28 6h42m 7h16m 19h44m 20h18m 20h18m 0.46
2021-10-29 6h43m 7h17m 19h43m 20h17m 20h17m 0h31m 0.37
2021-10-30 6h45m 7h19m 19h41m 20h15m 20h15m 1h39m 0.27
2021-10-31 6h46m 7h20m 19h40m 20h14m 20h14m 2h50m 0.18

Twilight calendar was generated using Cartes du Ciel (Sky Charts).

See my blog post: Twilight Zone for the definitions of the different periods of twilight here: https://bobmoler.wordpress.com/2018/09/27/.

NASA Calendar of Planetary Events

Date         Time ET  Event
Oct  1  Fr            Venus: 44.9° E
     1  Fr   9:08 am  Moon-Beehive: 3.3° S
     6  We   7:05 am  New Moon
     7  Th  11:50 pm  Mars Solar Conjunction
     8  Fr   1:28 pm  Moon Perigee: 363400 km
     9  Sa  12:12 pm  Mercury Inferior Conj.
     9  Sa   2:36 pm  Moon-Venus: 2.9° S
     9  Sa   3:35 pm  Moon Descending Node
    10  Su            S Taurid peak ZHR = 5 See IMO Calendar
    12  Tu   5:09 am  Moon South Dec.: 26.2° S
    12  Tu  11:25 pm  First Quarter
    14  Th   3:12 am  Moon-Saturn: 4.1° N
    15  Fr   5:58 am  Moon-Jupiter: 4.3° N
    16  Sa   9:24 am  Venus-Antares: 1.5° N
    20  We  10:57 am  Full Moon
    21  Th   7:30 am  Orionid Shower: ZHR = 20
    23  Sa   7:47 am  Moon Ascending Node
    24  Su  11:30 am  Moon Apogee: 405600 km
    25  Mo  12:59 am  Mercury Greatest Elongation: 18.4° W
    26  Tu  12:04 pm  Moon North Dec.: 26.3° N
    27  We   4:40 pm  Moon-Pollux: 2.8° N
    28  Th   4:05 pm  Last Quarter
    28  Th   5:32 pm  Moon-Beehive: 3.6° S
    29  Fr   5:59 pm  Venus Greatest Elongation: 47° E
Nov  1  Mo            Venus: 47° E

All event times are given for UTC-4 hr: Eastern Daylight Saving Time.

Sky Events Calendar by Fred Espenak and Sumit Dutta (NASA’s GSFC),
http://eclipse.gsfc.nasa.gov/SKYCAL/SKYCAL.html.

If you go to the above site, you can print out a list like the above for the entire year or calendar pages for your time zone.

Sun and Moon Rising and Setting Events

LU                  Ephemeris of Sky Events for Interlochen/TC
October, 2021    Local time zone: EDT
+-----------------------------------------------------------------------+
| DATE |  SUN     SUN  DAYLIGHT|   TWILIGHT*    |MOON  RISE OR    ILLUM |
|      |  RISE    SET    HOURS |  END    START  |PHASE SET** TIME FRACTN|
+=======================================================================+
|Fri  1| 07:41a  07:22p  11:41 | 08:25p  06:38a |      Rise 02:49a   23%|
|Sat  2| 07:42a  07:20p  11:38 | 08:23p  06:39a |      Rise 04:01a   15%|
+------+-----------------------+----------------+-----------------------+
|Sun  3| 07:43a  07:19p  11:35 | 08:21p  06:40a |      Rise 05:15a    8%|
|Mon  4| 07:44a  07:17p  11:32 | 08:20p  06:41a |      Rise 06:30a    3%|
|Tue  5| 07:46a  07:15p  11:29 | 08:18p  06:43a |      Rise 07:46a    0%|
|Wed  6| 07:47a  07:13p  11:26 | 08:16p  06:44a |New   Set  07:41p    0%|
|Thu  7| 07:48a  07:11p  11:23 | 08:14p  06:45a |      Set  08:06p    3%|
|Fri  8| 07:49a  07:09p  11:20 | 08:12p  06:46a |      Set  08:35p    9%|
|Sat  9| 07:51a  07:08p  11:17 | 08:11p  06:47a |      Set  09:09p   17%|
+------+-----------------------+----------------+-----------------------+
|Sun 10| 07:52a  07:06p  11:14 | 08:09p  06:49a |      Set  09:53p   26%|
|Mon 11| 07:53a  07:04p  11:11 | 08:07p  06:50a |      Set  10:46p   37%|
|Tue 12| 07:54a  07:02p  11:08 | 08:05p  06:51a |F Qtr Set  11:50p   48%|
|Wed 13| 07:56a  07:01p  11:05 | 08:04p  06:52a |      Set  01:00a   59%|
|Thu 14| 07:57a  06:59p  11:02 | 08:02p  06:54a |      Set  02:13a   70%|
|Fri 15| 07:58a  06:57p  10:59 | 08:00p  06:55a |      Set  03:25a   79%|
|Sat 16| 07:59a  06:55p  10:56 | 07:59p  06:56a |      Set  04:35a   87%|
+------+-----------------------+----------------+-----------------------+
|Sun 17| 08:01a  06:54p  10:53 | 07:57p  06:57a |      Set  05:43a   93%|
|Mon 18| 08:02a  06:52p  10:50 | 07:56p  06:58a |      Set  06:49a   97%|
|Tue 19| 08:03a  06:50p  10:47 | 07:54p  07:00a |      Set  07:54a  100%|
|Wed 20| 08:05a  06:49p  10:44 | 07:52p  07:01a |Full  Rise 07:07p  100%|
|Thu 21| 08:06a  06:47p  10:41 | 07:51p  07:02a |      Rise 07:29p   98%|
|Fri 22| 08:07a  06:46p  10:38 | 07:49p  07:03a |      Rise 07:53p   95%|
|Sat 23| 08:09a  06:44p  10:35 | 07:48p  07:05a |      Rise 08:22p   90%|
+------+-----------------------+----------------+-----------------------+
|Sun 24| 08:10a  06:42p  10:32 | 07:46p  07:06a |      Rise 08:56p   84%|
|Mon 25| 08:11a  06:41p  10:29 | 07:45p  07:07a |      Rise 09:38p   76%|
|Tue 26| 08:13a  06:39p  10:26 | 07:44p  07:08a |      Rise 10:29p   67%|
|Wed 27| 08:14a  06:38p  10:23 | 07:42p  07:10a |      Rise 11:27p   58%|
|Thu 28| 08:15a  06:36p  10:21 | 07:41p  07:11a |L Qtr Rise 12:31a   48%|
|Fri 29| 08:17a  06:35p  10:18 | 07:39p  07:12a |      Rise 01:39a   38%|
|Sat 30| 08:18a  06:33p  10:15 | 07:38p  07:13a |      Rise 02:50a   29%|
+------+-----------------------+----------------+-----------------------+
|Sun 31| 08:19a  06:32p  10:12 | 07:37p  07:14a |      Rise 04:03a   19%|
+-----------------------------------------------------------------------+
* Nautical Twilight
** Moonrise or moonset, whichever occurs between sunrise and sunset

Generated using my LookingUp for DOS program.

09/29/2021 – Ephemeris – Let’s search for the naked-eye planets for this week

September 29, 2021 Comments off

This is Ephemeris for Wednesday, September 29th. Today the Sun will be up for 11 hours and 47 minutes, setting at 7:26, and it will rise tomorrow at 7:39. The Moon, 1 day past last quarter, will rise at 12:40 tomorrow morning.

Let’s search for the naked-eye planets for this week. Venus should be visible in the southwestern evening twilight by 7:45 tonight. It will set at 8:59 pm. By 8 pm, Jupiter will be spotted in the southeastern sky. The Jupiter should be easy to spot at that hour. Saturn will be dimmer, but a bit higher and to its right. They will be visible into the morning hours, with Saturn setting first at 2:26 am, with Jupiter following at 3:49. Saturn’s rings can be seen in a spotting scope of about 20 power magnification. Though at that power the rings won’t appear separated from the planet, so Saturn will look like an elliptical disk. Jupiter’s four brightest moons are spread out, two on each side. They all might be visible in binoculars tonight.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.

Addendum

Venus in evening twilight

Venus in evening twilight at 7:45 pm, about a half hour after sunset tonight, September 29, 2021. Created using Stellarium.

Jupiter and Saturn in evening twilight

Jupiter and Saturn in the southeast at 8 pm. Created using Stellarium.

Waning crescent Moon

Low magnification view of the waxing crescent Moon as it would appear at 6:30 am tomorrow, September 30, 2021. Created using Stellarium.

Telescopic views of the naked-eye planets

Telescopic views of the bright planets (north up) as they would be seen in a small telescope, with the same magnification, this evening at 8 pm, September 29, 2021. Apparent diameters: Venus, 18.65″, 62.6% illuminated; Saturn 17.68″, its rings 41.18″; Jupiter, 46.41″. The ” symbol means seconds of arc (1/3600th of a degree.) Click on the image to enlarge it. Created using Cartes du Ciel (Sky Charts).

Planets and the Moon overnight tonight

The naked-eye planets and the Moon at sunset and sunrise on a single night, starting with sunset on the right on September 29, 2021. The night ends on the left with sunrise on the 30th. Click on the image to enlarge it. Created using my LookingUp program.

09/28/2021 – Ephemeris – Andromeda, a damsel in distress

September 28, 2021 Comments off

This is Bob Moler with Ephemeris for Tuesday, September 28th. Today the Sun will be up for 11 hours and 50 minutes, setting at 7:28, and it will rise tomorrow at 7:38. The Moon, at last quarter today, will rise at 11:46 this evening.

In the east at 9 this evening can be found a large square of stars, the Great Square of Pegasus the flying horse. The square is standing on one corner. What looks like its hind legs stretching to the left from the left corner star is another constellation, Andromeda the chained princess. She is seen in the sky as two diverging curved strings of stars that curve upward. She was doomed due to her mother, Queen Cassiopeia’s boasting, which angered the god Poseidon. She was rescued by the hero Perseus, a nearby constellation, riding his steed Pegasus. Andromeda’s claim to astronomical fame is the large galaxy seen with the naked-eye just above the upper line of stars. The Great Andromeda Galaxy is two and a half million light years away.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.

Addendum

Andromeda finder animation

Andromeda finder animation surrounded by the other constellations in her story, except the monster, which will rise later. Click on the image to enlarge it. Created using Stellarium and GIMP.

Andromeda Galaxy finder chart

Great Andromeda Galaxy finder chart. This image shows the galaxy almost to its fullest extent. In the finder animation above, the galaxy looks pretty much as it would to the naked eye. Click on the image to enlarge it. Created using Stellarium.

Astronomers often refer to this galaxy as M 31 for short. It was the 31st entry in Charles Messier’s catalog of objects that could be confused as being comets by comet hunters like himself. It was added in 1764. He didn’t care what these fuzzy objects were, just that they didn’t move against the background stars. Actually, M 31 is in the background. The stars are in the foreground, in our Milky Way Galaxy.

 

09/27/2021 – Ephemeris – The native peoples constellations of the Crane and the Moose

September 27, 2021 Comments off

This is Ephemeris for Monday, September 27th. Today the Sun will be up for 11 hours and 53 minutes, setting at 7:30, and it will rise tomorrow at 7:37. The Moon, 1 day before last quarter, will rise at 11:00 this evening.

The evening sky hosts two more of the constellations of the Anishinaabe native peoples of our area. Overhead, where the official constellation Cygnus the Swan is, or the Northern Cross is Ajijaak, the Sand Hill Crane flying northward through the Milky Way, wings outstretched, with its long legs trailing behind. In the eastern sky where the official constellation of Pegasus the flying horse is climbing the sky upside down is. His body is the Great Square, an informal constellation. To the Anishinaabe, it is the Mooz (Moose), who is upright. His magnificent antlers take up the dim official constellation of Lacerta the lizard between Pegasus and Cygnus. Also in the sky is Ojiig the Fisher, our Big Dipper, whose bloody tail will soon swoop down and paint the trees with their fall colors. (You can search for “Fisher” above right for his story, and his relevant appearances in autumn and late winter.)

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.

Addendum

Anishinaabe constallation of the Crane an Moose

The Anishinaabe constellations of Ajijaak, the Crane and Mooz (Moose) compared to the official International Astronomical Union (IAU) constellations. Created using Stellarium and GIMP.

The Ojibwe constellation art is part of the latest versions of Stellarium. Ojibwe (Anishinaabe) constellation art by Annette S Lee and William Wilson from Ojibwe Sky Star Map Constellation Guide, ISBN 978-0-615-98678-4.

09/24/2021 – Ephemeris – Capricornus, home this season to Jupiter and Saturn

September 24, 2021 Comments off

This is Bob Moler with Ephemeris for Friday, September 24th. Today the Sun will be up for 12 hours and 3 minutes, setting at 7:35, and it will rise tomorrow at 7:33. The Moon, halfway from full to last quarter, will rise at 9:26 this evening.

Nearly 2000 years ago the southernmost of the constellations of the zodiac was Capricornus the water goat. That’s why the latitude on the Earth where the Sun is overhead on the winter solstice is called the Tropic of Capricorn. Not anymore, Sagittarius, one constellation west, has that honor today*. Capricornus is large, but made up of dim stars. To me, it looks like a 45 degree isosceles triangle, long side up, but which all the sides are sagging. The constellation is found low in the south at 10 to 11 p.m. The image that is supposed to be represented by the stars is that of a goat whose hind quarters are replaced by a fish’s tail, not a mermaid but a mer-goat. This autumn, Jupiter is at the left end of Capricornus, with Saturn on the right.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.

The reason for the shift is lunisolar precession, which I talked about yesterday.

Addendum

Capricornus w/Jupiter & Saturn finder animation

Capricornus w/Jupiter & Saturn finder animation for 10 pm, September 24, 2021. Created using Stellarium and GIMP.

09/23/2021 – Ephemeris – The Earth’s axial tilt gives us our seasons

September 23, 2021 Comments off

This is Bob Moler with Ephemeris for Thursday, September 23rd. Today the Sun will be up for 12 hours and 6 minutes, setting at 7:37, and it will rise tomorrow at 7:32. The Moon, 3 days past full, will rise at 9:03 this evening.

The Earth has an axial tilt* of about 23 and a half degrees, which gives us our seasons. Because the Earth rotates on its axis, it has a slight equatorial bulge. Earth’s polar diameter is 7,900 miles (12,714 kilometers) while its equatorial diameter is 7,926 miles (12,756 kilometers), a difference of 26 miles (42 kilometers). The gravitational tug on that equatorial bulge by the Moon and Sun actually keeps the tilt stable, but does cause the Earth’s axis to precess like a top slowing down. It’s why Polaris will no longer be our North Pole star in centuries to come, just as it wasn’t in centuries past. It’s also why the constellations of the zodiac no longer align with the astrological signs of Ptolemy’s zodiac of the second century AD.

The astronomical event times given are for the Traverse City/Interlochen area of Michigan (EDT, UT-4 hours). They may be different for your location.

* Astronomers call axial tilt “obliquity” or “obliquity of the ecliptic”.

Addendum

The force causing precession

The Moon and Sun’s gravitational force act on the Earth’s equatorial bulge, attempting to cause the Earth to straighten up and fly right. Because the Earth is spinning, it acts like a gyroscope and the torque to straighten it up causes it to be applied 90 degrees away in the direction of the rotation causing the procession. Image credit: Open Course: Astronomy.

Precesssion of a spinning top

Precession of a spinning top: the spin axis traces the surface of a cone. The axis, in the case of the Earth, traces a circle of radius 23.5 degrees on the sky. Credit NASA.

Precesion animation

The 25,700-year cycle of precession traced on the sky as seen from near the Earth. The current North Pole star is Polaris (top). In about 8,000 years it will be the bright star Deneb (left), and in about 12,000 years, Vega (left center). The Earth’s rotation is not depicted to scale – in this span of time, it would actually rotate over 9 million times. Credit image: Tfr000, caption: Wikipedia.