Stellar Cartographies

Updated: Jan 14, 2020

Happy New Year fellow astronavigators!

First blog of the decade and let me start with wishing you all a happy new year! The coming decade promises a lot of advancements in astronomy and spaceflight, but will equally challenge us with social media issues and climate change.

The return of the seasons and of sunlight is being celebrated. Exactly after one revolution around the Sun, a year after just the same happened. Astronomy is used as the basis of our calendar. Let's take a look at the star map. How do we map out the celestial mechanics to understand seasonal climates, and later even to navigate the world with the stars?

As you all have noticed, our game makes use of such a classic star map, based on equatorial coordinates. This way of mapping the stars dates back to Hipparchus of Nicaea's astrometric measurements in the 2nd century BC. He was probably the inventor of the astrolabe and armillary sphere while he was making his famous star catalogue. The relation to the Earth axis rotation is key in under standing the 24h Right Ascension. Hipparchus is particularly famous for his accidental discovery of the precession of the equinoxes.

In this blog I want to discuss different ways of looking at a star map and different coordinate systems linked to it. In the board game, I tried to indicate groups of constellations that have common position features. This way sets can be collected. Three of these sets are linked to these coordinate systems. Whenever a set is a reference to a hemisphere, the celestial poles or even precession, the player has the opportunity to understand the equatorial coordinate system. The most important instrument to represent this system is the armillary sphere. But after a brief overview of the equatorial, and thus the classical, coordinate system, we take a look at the ecliptic coordinate system embedded in the set collections, which is important to understand the Solar System and its orrery designs, and we conclude with the galactic coordinate system. The latter is crucial to study star positions in the Milky Way, star population groups, stellar evolution, migration and proper motion.

a) WHAT?

Classic tilted base position is 00° which is equal to Equatorial coordinate system

Star Map for AstroNavigators, equatorial reference (Guilhem Bedos and Vincent Verhoeven)

b) HOW?

It is based on the common observation of day and night, which led to another insight of a spherical Earth instead of a flat Earth. The Earth axis and the equinoxes (where the apparent sky route of the Sun and the equator meet) were key for Hipparchus to compose his famous star map and catalogue. There is a strong connection with the latitudes and longitudes on the Earth globe, since it was derived from the celestial sphere. Equivalent to the Earth's equator is the celestial equator. (Right image from Wikipedia)

This forms the basis for celestial latitude measurement, expressed by the 24h Right Ascension (RA) notation from right to left, every stellar hour covers 15° of the celestial globe, parallel with the Earth's longitude. The big difference with the celestial coordinates, is that it is time related due to the Earth's rotation. That's why it has been difficult to determine the longitude position until (nautical) clocks were invented. The primary direction (0h) points from the Earth towards the constellation Pisces (as from the year 2000) where the Sun is at the vernal equinox (Spring) in March of the Northern Hemisphere. Due to the precession and the stars' proper motion, every 50 year new positions are fixed for a period of 50 year. Today's positions are fixed on J2000; indicating the year 2000 as the current reference.

The latitude was easily pinpointed with the horizon thanks to astrolabes and sextants, by using the object's celestial position, expressed by its +90°/-90° declination (Dec) in the sky. Declination notations are positive from the celestial equator to the celestial North Pole (0° to +90°) and negative from the celestial equator to the celestial South Pole (0° to -90°).

c) WHY?

This model helps us understand astronavigation on Earth and time-keeping (calendar). An important navigation device that was also used as time tracker, was the astrolabe. This portable device has been a status symbol for wealth and knowledge across the Greek and Persian world. Afterwards, the Portuguese had claimed the maritime version and together with the armillary sphere it became a symbol of discovery, before the sextant and octant took its place for exploring the seas. 

Vasco da Gama Leaving Lisbon Portugal, mural (by John Henry Amshewitz)
Nautical astrolabe design (by Diego Ribero, 1529)


The understanding of the Earth-axis mechanics have led to the understanding of Earth coordinates linked with our planet's dynamics in space, unlocking the intercontinental Age of Discovery (15th century) as soon accurate navigation instruments were invented.

Our society transformed into intercontinental proportions.

a) WHAT?

Classic tilted base position 23° (rounded) which is the plane for the Ecliptic coordinates

Star Map for AstroNavigators, ecliptic reference (Guilhem Bedos and Vincent Verhoeven)

b) HOW?

Ecliptic longitude measures the angular distance of an object along the ecliptic plane. Like RA in the equatorial coordinate system, the primary direction (0° ecliptic longitude) points from the Earth towards the constellation Pisces (J2000) where the Sun is at the vernal equinox (Spring) in March of the Northern Hemisphere. Because it is a counterclockwise system, ecliptic longitude is measured positive eastwards in the fundamental plane (the ecliptic) from 0° to 360°. The ecliptic North and South Pole are equally expressed as +90° and -90°. (Right image from Wikipedia)

c) WHY?

This model helps us understand astronavigation in the Solar System. It brought a cosmological revolution for human understanding and gave a better understanding of time-keeping, which resulted in several corrections (calendar leap years). Today it serves as a map for interplanetary discoveries and spaceflight.


The understanding of planetary mechanics was the start of the age of planetary discoveries (17th century), since the 1960s unmanned spaceflight brought us closer to these planets.

Orrery (

Based on Tycho Brahe's observations, Johannes Kepler could mathematically prove Nicolaus Copernicus' heliocentric model and clarify the retrograde (backwards) movement we observe of these planets (wandering stars). Galileo Galilei's telescopic observations have,  together with Kepler's mathematical discovery, also revolutionized the view that not everything revolves around the Earth. Jupiter is another object with its own revolving moons. Centuries following this insight and the use of telescopes, have led to the discoveries of more objects in the Solar System, like Uranus and Neptune. It is not until Newton's law of motions and gravity (17th and 18th century) converged with the jet propulsion advancement in the 20th century, that spaceflight emerged.

Today our society is ready to transform into interplanetary proportions while aiming at settling on the Moon and Mars.

Ecliptic plane and zodiac (
Retrograde movements of planets in the sky (
Orrery board for AstroNavigators, ecliptic reference (Guilhem Bedos and Vincent Verhoeven)
Planets and zodiac references (

a) WHAT?

Classic tilted base position is (rounded) 63° which is the plane for the Galactic coordinates

Star Map for AstroNavigators, galactic reference (Guilhem Bedos and Vincent Verhoeven)

b) HOW?

Galactic Coordinates (

The first galactic coordinate system was used by William Herschel in 1785. Several coordinate systems, each differing by a few degrees, have been used until after The Great Debate (between Harlow Shapley and Heber Curtis) about the size of the universe, and after the intergalactic discoveries of Edwin Hubble in the 1920s. These events stimulated the first attempt for an international system in the 1930s. In 1958, the International Astronomical Union (IAU) defined the galactic coordinate system more accurately. In the equatorial coordinate system, the north galactic pole (NGP) is defined at RA 12h 49m and Dec +27.4°, in the constellation Coma Berenices. Longitude 0° is the point towards the galactic centre with respect to the galactic plane. The galactic latitude is positive towards the north galactic pole and negative towards the south galactic pole. The galactic longitude is equally measured counterclockwise in the fundamental galactic plane from 0° to 360°. The ecliptic Galactic North and South Pole are equally expressed as +90° and -90°.

c) WHY?

This model helps us understand astronavigation in the Milky Way. It resulted from the 1920s cosmological revolution on a galactic scale, but the catalogue of stars was really evolved thanks to ESA's space telescopes Hipparcos (launch 1989) and Gaia (launch 2013).

Milky Way galactic coordinate system (Wikipedia)
Mapped Cepheid stars (Serge Brunier / ESO / Skowron et al.)


The understanding of interstellar mechanics was the start of the age of interstellar discoveries (20th century), during the 1990s the first extrasolar planets were discovered and opened the door the search of Earth 2.0 and the 21st century brought more discoveries of Earth-like planets, thanks to a.o., Kepler Space Telescope (launch 2009) in the solar neighbourhood. The Summer Triangle with Cygnus, Lyra and Aquila, is the average 90° direction our Sun is moving to in the Milky Way and was the first area for the Kepler telescope to scan the galactic plane for sun-like stars with exoplanets.

Our society has dreams of interstellar proportions!

Exoplanets in a galactic overview (NASA)
Milky Way orientation map (


Over 700 galaxies appear in this image of the Virgo Cluster, including the string of galaxies known as Markarian’s Chain at the center of the field. The Virgo Cluster, which sits 65 million light-years away, is near the centre of our Local Supercluster; their coincident positions are why it is called the Virgo Supercluster ( - Kees Scherer)
Virgo Supercluster, dominated by the Virgo Cluster, around which our Local Group of galaxies seem to revolve, observable in the constellation Virgo (Wikimedia Commons)

The next step would be Supergalactic coordinates. Because of its intergalactic nature, this is only relevant for cosmological observations. It centres the Virgo cluster of galaxies for gravitational dynamic flow purposes to analyse the Supercluster as a whole. Its relation with astronavigation eludes the observation scope of our board game.

Following this article on coordinate systems, there will be a small blog with a question to our followers and potential backers about how far we can go with the scientific accuracy versus gameplay, especially concerning observations.

In the next big blog however, we will look at the different spacesuits used in the game. An opportunity to take a brake of astronomy and concentrate ourselves on spaceflight engineering. Is it based on real spacesuits, did we get some inspiration elsewhere and why did we choose this model?

I hope you enjoyed this blog!

Write to you all soon...


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