The Dimmest Constellation

You are probably familiar with at least the names of the twelve constellations of the zodiac:


But are you familiar with the twelve constellations that have no stars brighter than 4th magnitude?

Coma Berenices
Corona Australis

All but two of these dim constellations are, at least in part, visible from southern Arizona; Chamaeleon and Mensa require a trip south to see.

The southern constellation Mensa, the Table Mountain (declination -70° to -85°) is a ghost of a constellation, exhibiting no star brighter than magnitude 5.1. That’s 17 times fainter than Polaris! In fact, that’s fainter than all the stars of the Little Dipper asterism! Mensa does have one claim to fame, however. The Large Magellanic Cloud, satellite galaxy of our Milky Way galaxy, straddles most of the border that Mensa shares with Dorado, the Swordfish.

Mensa is far and away the dimmest constellation. But Mensa is a small constellation, bested in size by 74 of the 88 constellations. So perhaps it is not too surprising that a small constellation is less likely to harbor a bright star. Another measure of faint, perhaps, is to determine which of these twelve constellations with no star brighter than 4th magnitude is largest. That might be more remarkable, because one is less likely to find no bright stars in a large area of sky than in a small area of sky. By this measure, Camelopardalis, the Giraffe, wins without a doubt. Camelopardalis is the 18th largest constellation, and yet contains no star brighter than magnitude 4.0. It is that empty region you might have not noticed midway between Capella and Polaris, best viewed at evening twilight’s end during the month of February each year.

Sagittarius Time Machine

The bright stars that outline our constellations beckon to us from a remarkably wide range of distances. Many of these stars are super-luminous giant stars and hot blue dwarf stars. More typical stars like our Sun—and the even more abundant red dwarf stars—are much too faint to see with our unaided eyes, unless they are only a few light years away. Thus many of the stars we see when we look up at the night sky are the intrinsically brightest ones, the “whales among the fishes.”

Trigonometric parallax directly provides us with the best estimate of the distance to each of these stars (provided they are not more than a few hundred light years away), and once you know the distance, it is easy to calculate when the light you are seeing tonight left each one of them. It is enjoyable to contemplate what was going on in Earth history when each star’s light began its long journey across interstellar space, the tiniest fraction of which is reaching your eyes as you look up on a clear night.

This article is the next in a series featuring the major stars of a prominent constellation. We turn now towards Sagittarius, which is currently crossing the celestial meridian at the end of evening twilight.

Below you will find a chart showing the constellation Sagittarius and the bright stars that define its outline. The official IAU-approved star names are listed, where available, or the Bayer designation. There’s a printer-friendly PDF version of this chart at the bottom of this article. There’s room for you to write in the year when the light we are currently receiving left the photosphere of each star, using the provided table (which is updated automatically to today’s date).

The “Teapot” asterism of Sagittarius

The table below contains all the relevant information. There are three tabs: Parallax, Distance, and Time. The first three columns of each tab show the star name, the Bayer designation, and the spectral type and luminosity class listed in SIMBAD.

On the Parallax tab, the parallax in millarcseconds (mas) is listed in column D, along with the uncertainty in the parallax in column E, and the year the parallax was published in column F. All are from SIMBAD. I will update these values as new results become available, but please post a comment here if you find anything that is not current, or is incorrect.

On the Distance tab, the parallax and parallax uncertainty for each star is used to calculate the range of possible distances to the star (in light years) in columns D through F. The nominal value given in column E is our current “best guess” for the distance to the star.

On the Time tab, the range of distances from the Distance tab are used to determine the range of years when the light we are seeing at this point in time would have left the star. The earliest year (given the uncertainty in parallax) is shown in column D, the most likely year in column E, and the latest year (given the uncertainty in parallax) in column F.

Here’s a printer-friendly PDF version of the Sagittarius chart where after printing you can enter the nominal year from column E of the Time tab next to the name for each star. The year values on the Time tab will update automatically to reference the current date.

Globulars Galore

So far, a total of 162 globular clusters have been discovered in our Milky Way galaxy.

Many of the recent globulars that have been discovered are either heavily obscured by intervening interstellar matter at visible wavelengths (and thus detectable only in the infrared), or they are so diffuse that they are difficult to detect against the field stars.

Here’s a list of the 88 constellations, and how many globulars have been found in each.

Milky Way Globular Clusters

46 of the 88 constellations harbor globulars (52%). Sagittarius contains the most globular clusters, 36, representing nearly 22% or about 1/5 of the total. This is perhaps not surprising as the center of our Milky Way galaxy (Sgr A*) is located at a distance of 26,673 ± 72 ly from our Solar System in the direction of Sagittarius near the Sagittarius-Ophiuchus-Scorpius border.

Only two other constellations host more than 5 globular clusters: Ophiuchus is in 2nd place with 25, and Scorpius comes in 3rd with 20. Together these three adjacent constellations, Sagittarius, Ophiuchus, and Scorpius, contain a total of 81 globular clusters, exactly half (50%) of all the known Milky Way globulars! Truly, then, the Sagittarius+Ophiuchus+Scorpius region can be called the “Realm of the Globulars”.

The northernmost globular cluster is Palomar 1 (Cepheus, α2000 = 3h33m19s, δ2000 = +79°34’55”), and the southernmost globular cluster is IC 4499 (Apus, α2000 = 15h00m19s, δ2000 = -82°12’50”).

NGC 6101
IC 4499

NGC 6981 (M72)
NGC 7089 (M2)
NGC 7492

NGC 6749
NGC 6760
Palomar 11

NGC 6352
NGC 6362
NGC 6397
FSR 1735

Palomar 2

NGC 5466

Canes Venatici
NGC 5272 (M3)

NGC 7099 (M30)
Palomar 12

NGC 2808

NGC 5139 (Omega Centauri)
NGC 5286
Ruprecht 106

Palomar 1

Whiting 1

ESO 37-01 (E3)

NGC 1851

Coma Berenices
NGC 4147
NGC 5024 (M53)
NGC 5053

Corona Australis
NGC 6541

Crater (Laevens 1)

NGC 6934
NGC 7006
Laevens 3


NGC 6205 (M13)
NGC 6229
NGC 6341 (M92)
Palomar 14

NGC 1261
Arp-Madore 1

NGC 4590 (M68)
NGC 5694
Arp-Madore 4

NGC 1904 (M79)

NGC 5897

NGC 5824
NGC 5927
NGC 5986

NGC 2419

NGC 6779 (M56)

NGC 4372
NGC 4833
Van den Bergh-Hagen 140 (BH 140)

NGC 5946
FSR 1716
Lynga 7

NGC 6171 (M107)
NGC 6218 (M12)
NGC 6235
NGC 6254 (M10)
NGC 6266 (M62)
NGC 6273 (M19)
NGC 6284
NGC 6287
NGC 6293
NGC 6304
NGC 6316
NGC 6325
NGC 6333 (M9)
NGC 6342
NGC 6355
NGC 6356
NGC 6366
NGC 6401
NGC 6402 (M14)
NGC 6426
NGC 6517
IC 1257
HP 1
Palomar 6
Palomar 15

NGC 6752

NGC 7078 (M15)
Palomar 13

NGC 2298


NGC 6838 (M71)
Palomar 10

NGC 6440
NGC 6522
NGC 6528
NGC 6540
NGC 6544
NGC 6553
NGC 6558
NGC 6569
NGC 6624
NGC 6626 (M28)
NGC 6637 (M69)
NGC 6638
NGC 6642
NGC 6652
NGC 6656 (M22)
NGC 6681 (M70)
NGC 6715 (M54)
NGC 6717
NGC 6723
NGC 6809 (M55)
NGC 6864 (M75)
Arp 2
Van den Bergh-Hagen 261 (BH 261)
Djorgovski 2 (Djorg 2)
Palomar 8
Sagittarius II (Laevens 5)
Terzan 5
Terzan 7
Terzan 8
Terzan 9
Terzan 10
Terzan 12

NGC 6093 (M80)
NGC 6121 (M4)
NGC 6139
NGC 6144
NGC 6256
NGC 6380
NGC 6388
NGC 6441
NGC 6453
NGC 6496
Djorgovski 1 (Djorg 1)
ESO 452-SC11
FSR 1758
Liller 1
Terzan 1
Terzan 2
Terzan 3
Terzan 4
Terzan 6
Tonantzintla 2 (Ton 2)

NGC 288

NGC 6712
Mercer 5

Serpens (Caput)
NGC 5904 (M5)
Palomar 5

Serpens (Cauda)
NGC 6535
NGC 6539
IC 1276

Palomar 3

NGC 6584

NGC 104 (47 Tuc)
NGC 362

Ursa Major
Palomar 4

NGC 3201

NGC 5634


Fundamental parameters of Galactic globular clusters (as of May 2021)
Accessed: November 29, 2021

A geometric distance measurement to the Galactic center black hole with 0.3% uncertainty
The GRAVITY Collaboration, R. Abuter, A. Amorim, M. Bauböck, J. P. Berger, H. Bonnet, W. Brandner, Y. Clénet, V. Coudé du Foresto, P. T. de Zeeuw
A&A, 625 (2019) L10

Like Sun, Like Moon

The Earth orbits the Sun once every 365.256363 (mean solar) days relative to the distant stars.  The Earth’s orbital speed ranges from 18.2 miles per second at aphelion, around July 4th, to 18.8 miles per second at perihelion, around January 3rd.  In units we’re perhaps more familiar with, that’s 65,518 mph at aphelion and 67,741 mph at perihelion. That’s a difference of 2,223 miles per hour!

As we are on a spinning globe, the direction towards which the Earth is orbiting is different at different times of the day.  When the Sun crosses the celestial meridian, due south, at its highest point in the sky around noon (1:00 p.m. daylight time), the Earth is orbiting towards your right (west) as you are facing south. Since the Earth is orbiting towards the west, the Sun appears to move towards the east, relative to the background stars—if we could see them during the day.  Since there are 360° in a circle and the Earth orbits the Sun in 365.256363 days (therefore the Sun appears to go around the Earth once every 365.256363 days relative to the background stars), the Sun’s average angular velocity eastward relative to the background stars is 360°/365.256363 days = 0.9856° per day.

The constellations through which the Sun moves are called the zodiacal constellations, and historically the zodiac contained 12 constellations, the same number as the number of months in a year.  But Belgian astronomer Eugène Delporte (1882-1955) drew up the 88 constellation boundaries we use today, approved by the IAU in 1930, so now the Sun spends a few days each year in the non-zodiacal constellation Ophiuchus, the Serpent Bearer. Furthermore, because the Earth’s axis is precessing, the calendar dates during which the Sun is in a particular zodiacal constellation is gradually getting later.

Astrologically, each zodiacal constellation has a width of 30° (360° / 12 constellations = 30° per constellation).  But, of course, the constellations are different sizes and shapes, so astronomically the number of days the Sun spends in each constellation varies. Here is the situation at present.



Sun Travel Dates


Sea Goat

Jan 19 through Feb 16


Water Bearer

Feb 16 through Mar 12


The Fish

Mar 12 through Apr 18


The Ram

Apr 18 through May 14


The Bull

May 14 through Jun 21


The Twins

Jun 21 through Jul 20


The Crab

Jul 20 through Aug 10


The Lion

Aug 10 through Sep 16


The Virgin

Sep 16 through Oct 31


The Scales

Oct 31 through Nov 23


The Scorpion

Nov 23 through Nov 29


Serpent Bearer

Nov 29 through Dec 18


The Archer

Dec 18 through Jan 19

The apparent path the Sun takes across the sky relative to the background stars through these 13 constellations is called the ecliptic.  A little contemplation, aided perhaps by a drawing, will convince you that the ecliptic is also the plane of the Earth’s orbit around the Sun.  The Moon never strays very far from the ecliptic in our sky, since its orbital plane around the Earth is inclined at a modest angle of 5.16° relative to the Earth’s orbital plane around the Sun.  But, relative to the Earth’s equatorial plane, the inclination of the Moon’s orbit varies between 18.28° and 28.60° over 18.6 years as the line of intersection between the Moon’s orbital plane and the ecliptic plane precesses westward along the ecliptic due to the gravitational tug of war the Earth and the Sun exert on the Moon as it moves through space.  This steep inclination to the equatorial plane is very unusual for such a large moon.  In fact, all four satellites in our solar system that are larger than our Moon (Ganymede, Titan, Callisto, and Io) and the one that is slightly smaller (Europa) all orbit in a plane that is inclined less than 1/2° from the equatorial plane of their host planet (Jupiter and Saturn).

Since the Moon is never farther than 5.16° from the ecliptic, its apparent motion through our sky as it orbits the Earth mimics that of the Sun, only the Moon’s angular speed is over 13 times faster, completing its circuit of the sky every 27.321662 days, relative to the distant stars.  Thus the Moon moves a little over 13° eastward every day, or about 1/2° per hour.  Since the angular diameter of the Moon is also about 1/2°, we can easily remember that the Moon moves its own diameter eastward relative to the stars every hour.  Of course, superimposed on this motion is the 27-times-faster-yet motion of the Moon and stars westward as the Earth rotates towards the east.

Now, take a look at the following table and see how the Moon’s motion mimics that of the Sun throughout the month, and throughout the year.


——— Moon’s Phase and Path ———


Sun’s Path





Mar 20






Jun 21






Sep 22






Dec 21






New = New Moon

near the Sun

FQ = First Quarter

90° east of the Sun

Full = Full Moon

180°, opposite the Sun

LQ = Last Quarter

90° west of the Sun


= crosses the celestial equator heading north


= rides high (north) across the sky


= crosses the celestial equator heading south


= rides low (south) across the sky

So, if you aren’t already doing so, take note of how the Moon moves across the sky at different phases and times of the year.  For example, notice how the full moon (nearest the summer solstice) on June 27/28 rides low in the south across the sky.  You’ll note the entry for the “Jun 21” row and “Full” column is “Low”.  And, the Sun entry for that date is “High”.  See, it works!

M31, M33, and the Milky Way

We live within a small cluster of at least 54 galaxies (most of them small), given the unassuming name “The Local Group” by Edwin Hubble (1889-1953) in 1936. The largest of these, M31 (the Andromeda Galaxy) is a barred spiral galaxy 2.5 million light years from Earth.  The second largest galaxy in the Local Group is our Milky Way, a barred spiral galaxy whose center lies about 26,000 light years away in Sagittarius.  The third largest galaxy in the Local Group is M33 (the Triangulum Galaxy), a spiral galaxy (possibly barred) located 3.0 million light years from Earth.  There are no other spiral galaxies in the Local Group.

M31 and our Milky Way are moving towards each other, and will pass through one another (or at least graze: shall we call it star grazing?) in about 4 billion years.

M33, however, is only about 860,000 light years from M31.  Isn’t M33 in even greater danger of colliding with M31?  The answer is no, we don’t think so, because M33 appears to be orbiting M31.  M33’s eventual role in the Battle of the Titans remains to be determined.