Concepts & Definitions
Cosmology is the study of the cosmos or the universe in its totality. In Zoroastrianism, this translates as the study in space and time of creation, its origin, purpose and goal.

The cosmos is the ordered system of the universe as a harmonious and integrated whole. It is a philosophical concept that lies at the heart of the concept of rita (rta) or asha. Rita and asha, the cosmic laws, are both the laws and the order that they bring to all existence - the order in nature on this earth, as well as the order that the ancients observed in the celestial sphere of the heavens.

In Zoroastrian cosmology, finite space and time were preceded by unlimited time and endless light. Zoroastrian astrology of the world is measured in ages and 'millennia' - the twelve ages being defined by the position of the Sun in a particular sector of the ecliptic and the specific constellation of the zodiac occupying the sector at that particular point in time. The duration of time when the Sun 'rules' or 'resides' in one of the twelve sectors is called an 'age' and at times erroneously a 'millenium' (Greater Bundahishn chapter 36). We will continue to develop this concept.

In Middle Persian, the cosmological horoscope of the world is called zaych-i gehan.

Understanding the Skies
Model of the celestial sphere.
Image credit: Celestial Sphere Model
Celestial Sphere
Spihr-e Gumezishnig
Zoroastrian astronomers understood that the various bodies of the cosmos were spherical or near spherical in shape. They also thought of the universe itself as spherical and the earth as a sphere. The manner in which they explained the phenomenon to a novice was to think of the cosmos as an egg and the earth as a yolk within the egg (Menog-i Kharad, 44.1-2 (tr. by West in SBE 24.85). Middle Persian texts contain the term 'celestial sphere'.

Dinkard Notes that "In the religion, the figure of the earth being round, the half of that earth of seven karshwars is said to be 3 1/2 karshwars; and owing to the earth being round the sun in its motion always shines for the beauty, the refulgence and the luster of half the earth."

By the reckoning of ancient Zoroastrians, the spherical earth was surrounded by vayu, an atmosphere, (Av. vayu Phl. vai; Av. thwasa Phl. spihr) beyond which was the sky, (Av., O.P. and Phl. asman). There was also the concept of different stations in the universe: the Moon, Sun and star stations (cf. Arda Viraz Nameh, and Zadspram). 1.29 of Zadspram also states "the sky extends as much as one-third beyond the star station." We are not concerned here with measurements but rather the concept of a multi-dimensional sky rather than the uni-dimensional shell of an egg used in the explanatory metaphor. The Bundahishn 7.1-2 states that beyond the stations are the endless lights, meaning the empyrean spiritual dimension.

In Zoroastrian cosmology, the celestial sphere resulted from from the creation of limited space from the endless lights (infinite space).The sphere is called spihr-e gumezishnig (often translated as 'mixture' but of uncertain meaning).

Constellations & Zodiac
Dwazdah-Akhtaran & Akhtaran
Some stars of the night sky stand out as brighter than others and appear to form a recognizable grouping or pattern called a constellation (akhtar). in 1922, 88 constellations (akhtaran)were endorsed by the International Astronomical Union (IAU).

In the Zoroastrian and several other traditions, twelve of the various constellation in the zodiac were identified as lying on the path followed by the Sun during an entire year (a solar year). These twelve constellations (dwazdah-akhtaran) of the zodiac were associated with animals, people or objects, given names and made up the zodiac. They were also considered bayan, givers of beneficence while the planets could portend negative change and harm (for a further discussion on the zodiac, please see the page on the Zodiac).

Distances in the celestial sky are measured in degrees (khurdak/sus/sush). Like a month, each sector of the ecliptic band around the earth (see below) defined by the path of the Sun and occupied by a constellation of the zodiac consists of 30°.

According to the Greater Bundahishn (GB 2.6) the cosmos was fashioned in the semblance of a (solar) year and that each of "the twelve constellations, like twelve months, have thirty degrees just as every month has thirty days." The balance of the last five days in the year (six in a leap year) are considered as particular auspicious and were traditionally set aside as an annual holiday just before the New Year celebrations on the vernal equinox (March 21 in the northern hemisphere). The treatment of the five/six days as part of the zodiac sign or houses are unclear. The Bundahishn calls them 'stolen' days (GB 1.21). However, according to the system of a full degree a day, a year should have 365 degrees and not 360 degrees.

Orbits of the planets around the Sun.
The flat plane of the orbits (other than Pluto) extending
into the universe is one way of understanding the ecliptic.
The ecliptic is the centre line of the path traced by the Sun in the sky. Stated differently, the ecliptic is the centre line of the zodiacal or ecliptic belt that encircles the Earth. The boundaries of the belt are 12° or 8° (depending on the convention used) on either side of the ecliptic.

The ecliptic is so named because eclipses occur only when the Moon is on or near this path. In other words, the sun, Moon and Earth are all on one plane. Most of the other planets in the solar system (other than Pluto) also revolve around the Sun in orbits that lie approximately within the ecliptic belt. That is because most of the planets (other than Pluto) orbit around the Sun on approximately one flat plane. The ecliptic can also be thought of as the plane - the flat disk or plate - formed by the earth's orbit around the Sun and then extending into the universe to the stars that form the constellations. The orbits of the seven ordinarily visible planets would lie on this plane.

The Zodiacal or Ecliptic Belt around the earth.
The constellations (clusters of stars)
are represented by symbols and names
Zodiacal / Ecliptic Belt
If it were possible to see the stars at daytime, the path traced by the Sun in the sky around the year would contain the twelve constellations that make up the Zoroastrian zodiac. The width of the constellations determine the width of the band or belt. The band that contains these constellations as well as the path of the Sun is called the zodiacal or ecliptic belt. By one standard the belt is 24° wide, and by another 16° wide.

If we were at the equator during an equinox, the belt would run from east to west and pass directly overhead. Then, with each passing day, the orientation of the belt would shift north or south and tilt in the process.

Sun & Ascendant Signs
When the Sun rises above the eastern horizon, it will have a constellation of the zodiac behind it. On any particular day, that name of the zodiac constellation in which the Sun is 'housed' is the Sun Sign (and this applies to the day a person is born).

At any particular time of a day, a zodiac constellation (sign) - or for that matter any notable star, say Sirius - will be rising above the eastern horizon - even if it is daytime and the bright sky hides the stars. The zodiac sign rising over the eastern horizon at time of a day is the Ascendant Sign (and this applies to the time a person is born). The Ascendant Sign is also called the Rising Sign which is a far more understandable term. The process of a sign or star rising in the eastern sky is called Ascendancy i.e., the sign or star is in Ascendancy meaning it is just in the process of rising above the eastern horizon. The Ascendancy of a Rising Sign begins to end just before the other sign appears.

We can only presume that allowances will need to be made for mountains and other features that delay the visual sighting of a sign as compared to when it would have been seen over a featureless plain.

Heliacal Rising / Ascendancy
Heliacal rising and setting. Credit: Shepard Simpson at
Helios means Sun in Greek and heliacal means 'near/with the Sun'.

The heliacal rising (ascendancy) of a star or another heavenly body occurs when after some days of invisibility because of sunlight, the body can very briefly (for say a minute of two) be seen glimmering just before sunrise on the eastern horizon makes it invisible again. This phenomenon occurs on just one day and is repeated on the same day every (solar) year in the usual life span of human beings - making it a way of marking a date in a solar year.

The heliacal rising of Tishtar, Sirius, around July 1, was used by ancient Zoroastrians to mark the beginning of the rain season. The day itself is called the day of Tir in the month of Tir, and the festival celebrated on that day is called Tirgan. For the legends surrounding the day, please see Tirgan at Zoroastrian Heritage.

[Note: In ancient times, while most cultures used a lunar based (primarily with some exceptions) calendar, the Zoroastrians used a solar seasonal calendar (months called 'mah' are based on a lunar cycle of 30 days, but the days of the calendar are not related to phases of the moon) and which was introduced to the West via  a related religion, Mithraism, and via Greek-Latin accounts of Zoroastrian practice. In lay language, all Zoroastrians days of note are determined by solar-based days and none by lunar-based days. The first day of the Zoroastrian year, the spring equinox, was determined in ancient times by the position of the rising Sun on the eastern horizon and not by positions of the stars or phases of the Moon. As such it was self-correcting. Leap-days were introduced later. The orthodox Zoroastrian calendar may be the world's only ancient perpetual solar calendar still in use today (see the Calender page at Zoroastrian Heritage).]

For a computer generated example of Sirius' heliacal rising or ascendancy, click here (Quick Time required).

Because the star appears to 'overtake' the Sun, it will therefore rise earlier than the Sun every subsequent day after the heliacal rising. It does so at the rate of about 1° or four minutes (in time) per day. At dawn, the star will have moved further along on its orbit, first rising and then setting towards the western sky. Eventually, it will have set in the western sky before dawn. In the process of 'overtaking' the Sun once again, the star will remain invisible entirely for some days obliterated by the glow of the Sun from which it will re-emerge. Its first brief sighting just before the Sun's glow makes it invisible is the heliacal rising.

A heliacal setting is the opposite phenomenon.

Start of the Zoroastrian & Modern-Western Day
Modern Western measurement of time has a day starting at midnight (a relatively recent development). The Zoroastrian day starts of daybreak - at dawn - and therefore in the Zoroastrian system, the hour before daybreak would be the last hour of the previous day while in the Western system it would be in the same day as the hour of daybreak.

Celestial Equator
The celestial equator is the projection of the earth's equator onto the celestial sphere. Because of the tilt of the earth, during winter in the northern hemisphere, the celestial equator is south of the ecliptic, while during the summer it is to the north of the ecliptic.

Obliquity of the Ecliptic
The celestial equator is inclined at an angle of 23°27’ (say 23°) to the ecliptic circle. This angle is known as the obliquity of the ecliptic. It is decreasing at the rate of 48 seconds of an arc degree in each century and will decrease for several millenniums until it reaches 22°54’, after which it will again increase. This movement is slight enough to think of the angle as a constant 23°.

The meridian is the line that run from the celestial north (visually determined by the north star) though the zenith - the point directly overhead. The meridian line is conceptually the same as a longitudinal line (see below). The main meridian is the meridian that runs through the meeting point of the ecliptic and the celestial equator.
The zenith, meridian and an introduction to how the coordinates of a heavenly body are determined in the sky.
Image credit:

Nodes or Equinoxes
The two points at which the ecliptic intersects the celestial equator are called nodes or equinoxes. The Sun is found at the intersection called the vernal equinox about March 21 and at the autumnal equinox about September 23.

Halfway on the ecliptic between the equinoxes are the summer and winter solstices. The Sun arrives at these points about June 21 and December 22, respectively.

The names of the four points of the equinoxes and solstices correspond to the seasons beginning in the northern hemisphere on these dates.
Solstice lines. Image credit: Vedic Astrology

Zoroastrian literature divides the sky into thirteen sectors: E, NEE, NNE N, NNW, NWW, W, WWS, WSS, S, SSE, SEE and Central. The outer sectors resemble the twelve signs of the zodiac when placed on a square chart. Each of the directional sectors has a chief star.

In modern astronomy, the ecliptic is used as the base circle for a system of coordinates called the ecliptic system.

Astronomers give stars coordinates within the celestial sphere or globe to locate them in the same manner as geographers use latitude and longitude coordinates to locate places on the Earth.
 Placing a grid system on the celestial sphere is the first step to establishing coordinates for a heavenly body in the sky. Image credit:

Celestial Coordinates: Equinox, longitude, equator, latitude.
Image credit: Astronomy 161
Right Ascension or Celestial Longitude (CA, a, alpha, α) is the celestial equivalent of longitude. Longitudinal lines run up-down and celestial longitude is measured east or right of the vernal equinox.

Declination or Celestial Latitude is celestial equivalent of latitude. Latitudinal lines run across the globe. Celestial latitude is measured north or south of the ecliptic.

Measurement of Longitude and Latitude (Dec, d, delta, δ)
Longitude and latitude are measured as angles made by lines drawn from two points on the sphere or globe to the centre of the earth.

Right ascension (RA or α) or celestial longitude is the angle made by lines drawn from the vernal equinox and a celestial body. The right ascension angel is denoted by the Greek alphabet alpha α.

RA (α) is measured eastwards from the vernal equinox. Longitudinal lines have a constant angle to the equinox and can be drawn, say, 15 degrees apart since the Sun takes one hour to pass through a 15 degree sector. In this case, the longitude number is 1h (15°), 2h (30°) & and so on running eastward around the celestial sphere to 23h. The vernal equinox is 0h ( or 24h/360°).

Declination (Dec or δ) or celestial latitude is the angle made by lines drawn from the ecliptic and a celestial body. It is measured north (a positive angle) or south (a negative angle) of the ecliptic. latitudinal lines have a constant angle to the ecliptic and can be drawn, say, 10 or 15 degrees apart.

Degree Measurement
In astrology, angles as well as time can be measured in degrees. Portions of a degree can be written in two ways: Using minutes and seconds or the decimal system.

The coordinates of a heavenly body M87 can be written as (a,d/α,δ) = 12:30:49, +12:23:07 or 187.705, +12.39619

As we have noted above, since the Sun moves around the full ecliptic circle of 360 degrees in 24 hours, time can be expressed in degree equivalents: 24hr=360° and 1hr=15°.

Aspect symbols and angles in Astrology. Image credit: Elsa Elsa
Aspect is an angle the planets make to each other on a horoscope chart.

Aspects are measured by the angular distance made at the centre of the earth along the ecliptic in degrees and minutes of celestial longitude between two points. The method of aspect notation is for example, Pluto at 20 Virgo 13, which means that Pluto's position relative to Virgo is 20 degrees and 13 minutes.

In astrology, only certain angular relationships are regarded as aspects and these relationships are named by the manner in which the planets are thought to work together when they are in a particular aspect.

Aspects (names on the outside of the circle), zodiac names and symbols (in the circle belt),
aspect signs and angles (inside of the circle)
Allowable aspect relationships have intrinsic astrological qualities and are categorized as harmonious, disharmonious or neutral. The aspects are further labelled as harmonic, or dynamic.

Aspect relationships have a certain amount of 'play' or margin in which they 'work', that is, in which they assert their assigned quality. This margin of 'play' is defined by an orb.

An orb is the area around an object that determines its sphere of influence. This sphere of influence is different for different heavenly bodies and is measured in arc degrees on the celestial sphere.

The table below (based on figures from Astrology Foundation) gives the size of an orb in arc degree around a heavenly body (planet, Sun and Moon) as determined by medieval astrologers based on the Persian system. There is considerable disparity in opinions about the size of an orb.

Obscurity Limit (Firmicus II.ix)
Orb Radius
(Al-Biruni v.436)
Orb Radius
(Ibn Ezra)

The above figures are for the radius of an orb. The extent of the orb on both sides of a body, namely the diameter, is twice the radius.

The table above includes figures that 4th century astrologer Julius Firmicus Maternus used to determine by how many degrees planets become morning stars, which rise before the Sun, or evening stars which appear after the Sunset. Heliacal is the term used to describe the rising or setting of a heavenly body that occurs at the same time as the rising or setting of the sun, because of their near conjunction or closeness.

It appears that the size of the orbs of the Sun and Moon have been arrived at from the distance at which other bodies become obscure when they are close to the Sun and Moon when both are rising or setting - during the heliacal phase. In other words, the Sun's orb of 15° – 17° is about the distance at which planets disappear from view when they enter into a conjunction with the Sun. The Moon’s orb of around 12° is also the distance that which separates the Sun and Moon when a new crescent Moon reappears after a conjunction of the two. The Moon becomes visible at a lesser distance than the planets because it is a more luminous body. The other factor that could have influenced the size of the orb is the body's perceived strength and influence, Jupiter, Saturn and even Mars being those with superior strength.

Because of the different size of the orbs, it is possible for one body to be in the orb of another with a larger orb while that larger orb body is not in the orb of the first with a smaller reach.

With respect to an aspect, an orb is the the margin of leeway on both sides of an aspect for the aspect to be operational astrologically.

Astrology considers that the tighter the orb, the more intense is the interplay between two planets.

Conjunction Aspect
When planets come within ten degrees of one another, they are said to be in a conjunction aspect. If the conjunction aspect is within 1-3 degrees, it is considered to be a tight and rare conjunction.

However, some astrologers feel that it is incorrect to call a conjunction an aspect, the reason being that conjoining planets do not 'look at each other'. Instead of being in a position across from one another where they can see one-another as they are thought to be doing in an aspect, in a conjunction they meet instead.

Sidereal & Tropical
Sidereal means relating to the stars, especially measured with reference to the apparent motion of the stars. The word comes to us directly or via French from the Latin sidereus, from sidus "star".
The different between sidereal and tropical (solar) reckoning of time. In the example above, a solar day is measured by determining when the Sun is overhead (or at a specific point in the sky). Sundials measure solar time and a day. A sidereal day is measured by determining when a particular star is overhead (or at a specific point in the sky). Observatory structures measure sidereal time at night. Since stars are much further away from the earth than the Sun, the line of vision to a star is almost parallel even after a shift by the earth in its orbit. However, the line of vision to the Sun becomes slightly angular when compared to the first sighting (A above).The star is therefore sighted about 4 minutes before the Sun and the sidereal day becomes slightly shorter. In astrology, the sighting of the Sun is replaced by measuring the movement of the equinox lines. Image credit: Encarta.
This diagram allows us to conceptualize the sighting of a star many, many time further away than the Sun - so far away that for all practical purposes the lines of sight a day apart appear to be parallel. Image credit:
Another way of looking at the difference between a sidereal
and solar (civil) sighting used to measure the passage of
time. Keep in mind that the star used for the sighting in the
diagram above is so far away compared to the Sun that
the line of sight to the star appears to be two parallel lines.
Image credit:
The solar and sidereal days in length
and showing the positions of the Sun and
star after a day. Image credit: The Full Wiki

Sidereal year is the time it takes the Earth to make one revolution around the Sun with reference to fixed stars, equal to 365 days, 6 hours, 9 minutes, 9.5 seconds
Sidereal month is the time it takes for the Moon to make one revolution around the Earth in relation to a given star. It is equal to 27 days, 7 hours, 43 minutes, 4.5 seconds.
Sidereal day is a day calculated by using a star's position, i.e., the time it takes for the Earth to make one complete revolution in relation to a given star, and is equal to 23 hours, 56 minutes, 4.1 seconds.

In astrology, tropical means relating to the Sun.
A tropical or solar year is the time taken for the Earth to move around the Sun, equal to 365 days, 5 hours, 48 minutes, 45.51 seconds. Also called astronomical year and the equinoctial year. Equinoctial means relating to the celestial equinox(es) and equator.
A tropical or solar day is what we commonly refer to as a day of 24 hours.

Sidereal astrology and tropical astrology are the two types of astrological systems. While both divide the ecliptic into a number of 'signs' named after constellations, the sidereal system defines the signs based on fixed stars, and the tropical system defines it based on the position of vernal equinox (i.e. the intersection of the ecliptic with the celestial equator). Because of the precession of the equinoxes, the two systems do not remain fixed relative to each other but drift apart by about 1.4 arc degrees per century. As a result, the signs drift along the ecliptic. While at one time the vernal equinox started in the sign of Aries, that is no longer the case.

Western astrology based on the system introduced Ptolemy in the 2nd century CE is tropical, while Hindu astrology primarily uses a sidereal system. Zoroastrian (Persian) astrology based on the Zoroastrian calendrical method of measuring time which is primarily solar but which also uses both systems in continuous harmony. Other systems may need periodic adjustments.