Comparing Two Birth Charts by Degree: How to Find Synastry Aspects

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Synastry aspects are found with simple arithmetic. You take one planet from each chart, turn both positions into a single number on the 360° circle, measure the gap, and see whether that gap is close enough to one of the five major aspect angles. Done carefully, the method catches contacts that a sign-by-sign comparison misses, and it shows which contacts depend on a birth time you may not have. This page walks through it with a real pair of charts. What each contact can mean is covered in the synastry basics hub and the contact guides linked below.

Educational guide: Astrologist.com does not currently calculate synastry, inter-chart aspects, house overlays or composite charts. The compatibility tool provides a sign-level estimate based on the Sun, Moon and Rising signs; known birth times are required for that tool. The contacts discussed here must be identified separately from the two charts' actual positions.

Step 1: get each person's positions

Generate one chart per person with the free birth chart. The table under the wheel lists every planet with its sign, degree and minute, for example "Venus 8°41' ♉". Write the positions down for both people before you start. The worked example on this page uses two example charts calculated with the site's own natal routines:

• Person A: 14 March 1988, 07:30, Amsterdam. Sun 23°54' Pisces, Moon 0°49' Aquarius, Venus 8°41' Taurus, Mars 14°03' Capricorn, Saturn 1°55' Capricorn.
• Person B: 2 August 1990, 21:10, London. Sun 10°16' Leo, Moon 26°58' Sagittarius, Venus 16°31' Cancer, Mars 13°48' Taurus.

Step 2: convert each position to longitude

Each sign covers 30° of the circle, starting from 0° Aries. Add the degrees within the sign to the starting point of that sign:

A's Sun at 23°54' Pisces becomes 330° + 23°54' = 353°54'. B's Moon at 26°58' Sagittarius becomes 240° + 26°58' = 266°58'. Keeping degrees and minutes avoids rounding slips; if you prefer decimals, divide the minutes by 60 (54' is 0.90°).

Step 3: measure the gap

Subtract the smaller longitude from the larger. If the answer is more than 180°, subtract it from 360°, because the short way round the circle is the one that counts.

353°54' − 266°58' = 86°56'. That is less than 180°, so the gap between A's Sun and B's Moon is 86°56'.

The wrap-around matters near 0° Aries. A planet at 355° and one at 3° look 352° apart on paper, but going the short way they are only 8° apart.

Step 4: compare the gap with the aspect angles

Under the convention used in these guides, a gap counts as an aspect when it falls inside one of these ranges:

The 86°56' gap falls in the square range, 3°04' short of exact, so A's Sun is square B's Moon.

These are the orbs the Astrologist.com calculator uses for aspects within one natal chart. On this page they serve only as the convention for educational examples; the software does not calculate synastry. Plenty of astrologers work with other orbs between charts, wider for the Sun and Moon in particular. If you use a different set, say which one, and apply it the same way to every pair. The general idea of an orb is explained in aspect orbs, and the five angles themselves in major aspects.

Step 5: check the other direction

Now swap roles: B's Sun against A's Moon. B's Sun at 10°16' Leo is 130°16'; A's Moon at 0°49' Aquarius is 300°49'. The gap is 170°33', which is 1°27' short of the 172° needed for an opposition. Under this convention there is no contact in that direction. An astrologer who allows a wider orb for the Sun and Moon might count it, which is exactly why the convention has to be stated before the results are read.

Repeat the measurement for every pair you are interested in. A full grid of the ten planets in each chart gives 100 pairs. For A and B, 39 of those 100 pairs fell within an orb. That is a normal number: over 20,000 compared pairs of charts with dates from 1950 to 2005, the average was 33.3.

Contacts that cross a sign boundary

Signs are not part of the measurement, and that is where many quick comparisons go wrong. A's Saturn at 1°55' Capricorn is 271°55'. B's Moon at 26°58' Sagittarius is 266°58'. The gap is 4°57', a conjunction, even though the two planets are in different signs. Someone matching signs only would see a Sagittarius Moon and a Capricorn Saturn and move on.

The reverse also happens: two planets in signs that usually form a trine can be too far apart by degree to count. A planet at 1° Aries and one at 29° Leo are in two fire signs but 148° apart, which is no major aspect at all.

Step 6: check whether a Moon contact depends on the birth time

The Moon moves roughly 12° to 16° in a day, so its position depends on the hour. Before you rely on a Moon contact, find out how much of the birth date supports it. Run the free chart twice for the person whose Moon it is, once at 00:00 and once at 23:59 of their birth date. These test charts treat each time as known, so ignore their Rising sign and houses; you only need the two Moon positions. The real Moon lies somewhere between them, because the Moon never moves backward.

For B, born on 2 August 1990 in London, the Moon ran from 16°22' to 28°24' Sagittarius over the day. The exact square to A's Sun falls at 23°54' Sagittarius, and every position in that range is within 8° of it. So A's Sun square B's Moon would hold even if B's birth time had been lost.

The other direction behaves differently. A's Moon on 14 March 1988 in Amsterdam ran from 26°17' Capricorn at 00:00 to 10°55' Aquarius at 23:59. It came within 8° of opposing B's Sun at about 09:52. A was born at 07:30, so there is no contact, but a birth two and a half hours later would have produced one.

When the time is unknown, the free chart draws the Moon at local noon as a date reference only. That noon position cannot establish a Moon contact on its own. In a check of 6,000 random pairs of dates, the noon Moon showed a conjunction with the other person's Sun in 257 cases, and in 207 of them, about four in five, the conjunction failed at some hour of that birth date. Without a time, report a Moon contact as possible, certain or impossible over the day, not as a fact taken from noon. The Moon without a birth time explains the same range for Moon signs.

Angles and house overlays need more than this check: without a reliable time and place, they are unknown.

Applying the method

With a list of contacts in hand, the contact guides explain what astrologers look for in the most discussed pairs:

• Sun-Moon contacts
• Venus-Mars contacts
• Moon-Moon contacts
• Moon-Venus contacts
• Saturn contacts with the Sun, Moon, Venus and Mars

The arithmetic tells you which contacts exist under a stated convention. It does not tell you how two people get on. Treat the list as material for reflection and conversation, read alongside each person's own chart.

Related articles

• Synastry Basics
• Synastry House Overlays
• House Overlays and Birth Time
• Aspect Orbs
• Major Aspects
• What Your Chart Can Show Without a Birth Time
• All compatibility guides