Scorpius Time Machine

One remarkable aspect of looking up at the bright stars that outline our constellations is that those stars are often at vastly different distances. The super-luminous giant stars and the hottest blue dwarf stars can be seen much further away than the much more common cooler dwarf stars like our Sun. Because of this, when we look up at the night sky with the unaided eye, many of the stars we are seeing are of the super-luminous variety, the “whales among the fishes”.

Once you know the distances to these stars, 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 represents the first in a series that will feature the distances to the major stars of a prominent constellation. We begin with Scorpius, which is currently crossing the celestial meridian at the end of evening twilight.

Below you will find a chart showing the constellation Scorpius 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, where the white text boxes have room for you to write in the nominal year when the light we are currently receiving left the photosphere of each star. The base chart was generated using SkySafari 6 Pro.

Scorpius

Next you’ll find a table that 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 value in millarcseconds (mas) is listed in column D, along with the mean error of 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 error for each star is used to calculate the range of distance to the star (in light years) in columns D through F, with the nominal value given in column E being our current “best guess” for the distance to the star.

On the Time tab, the range of distance from the Distance tab are used to calculate 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 Scorpius chart where you can enter the nominal year from column E of the Time tab for each star. The year values on the Time tab will update automatically to reference the current date. I’ve used the “Replace Color” adjustment feature in Adobe Photoshop to lighten the background color so your printer will save on ink when printing out the full-page chart.

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”).

Apus
NGC 6101
IC 4499

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

Aquila
NGC 6749
NGC 6760
Palomar 11

Ara
NGC 6352
NGC 6362
NGC 6397
ESO-SC06
FSR 1735

Auriga
Palomar 2

Boötes
NGC 5466

Canes Venatici
NGC 5272 (M3)

Capricornus
NGC 7099 (M30)
Palomar 12

Carina
NGC 2808

Centaurus
NGC 5139 (Omega Centauri)
NGC 5286
Ruprecht 106

Cepheus
Palomar 1

Cetus
Whiting 1

Chamaeleon
ESO 37-01 (E3)

Columba
NGC 1851

Coma Berenices
NGC 4147
NGC 5024 (M53)
NGC 5053

Corona Australis
NGC 6541

Crater
Crater (Laevens 1)

Delphinus
NGC 6934
NGC 7006
Laevens 3

Eridanus
Eridanus

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

Horologium
NGC 1261
Arp-Madore 1

Hydra
NGC 4590 (M68)
NGC 5694
Arp-Madore 4

Lepus
NGC 1904 (M79)

Libra
NGC 5897

Lupus
NGC 5824
NGC 5927
NGC 5986

Lynx
NGC 2419

Lyra
NGC 6779 (M56)

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

Norma
NGC 5946
FSR 1716
Lynga 7
RLGC 1

Ophiuchus
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

Pavo
NGC 6752

Pegasus
NGC 7078 (M15)
Palomar 13

Puppis
NGC 2298

Pyxis
Pyxis

Sagitta
NGC 6838 (M71)
Palomar 10

Sagittarius
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)
2MS-GC01
2MS-GC02
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
UKS 1
VVV-CL001

Scorpius
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)

Sculptor
NGC 288

Scutum
NGC 6712
Mercer 5
RLGC 2

Serpens (Caput)
NGC 5904 (M5)
Palomar 5

Serpens (Cauda)
NGC 6535
NGC 6539
IC 1276

Sextans
Palomar 3

Telescopium
NGC 6584

Tucana
NGC 104 (47 Tuc)
NGC 362

Ursa Major
Palomar 4

Vela
NGC 3201

Virgo
NGC 5634

References

Fundamental parameters of Galactic globular clusters (as of May 2021)
https://people.smp.uq.edu.au/HolgerBaumgardt/globular/
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.

Constellation

Description

Sun Travel Dates

Capricornus

Sea Goat

Jan 19 through Feb 16

Aquarius

Water Bearer

Feb 16 through Mar 12

Pisces

The Fish

Mar 12 through Apr 18

Aries

The Ram

Apr 18 through May 14

Taurus

The Bull

May 14 through Jun 21

Gemini

The Twins

Jun 21 through Jul 20

Cancer

The Crab

Jul 20 through Aug 10

Leo

The Lion

Aug 10 through Sep 16

Virgo

The Virgin

Sep 16 through Oct 31

Libra

The Scales

Oct 31 through Nov 23

Scorpius

The Scorpion

Nov 23 through Nov 29

Ophiuchus

Serpent Bearer

Nov 29 through Dec 18

Sagittarius

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 ———

Date

Sun’s Path

New

FQ

Full

LQ

Mar 20

EQ

EQ

High

EQ

Low

Jun 21

High

High

EQ

Low

EQ

Sep 22

EQ

EQ

Low

EQ

High

Dec 21

Low

Low

EQ

High

EQ

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

EQ

= crosses the celestial equator heading north

High

= rides high (north) across the sky

EQ

= crosses the celestial equator heading south

Low

= 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!