House overlay

Meaning

A house overlay takes the planets of one chart and places them in the houses of another. If her Venus falls in his 7th house, her Venus is said to "fall in" or "occupy" his house of partnership, and astrologers read her Venus as touching that part of his life. It helps to speak of a host, whose houses are used, and a guest, whose planets are placed in them.

Overlays run both ways, and the two directions are different: her planets in his houses and his planets in her houses are two separate lists. A full comparison looks at both.

Why the host's birth time matters. The planets' zodiac positions change slowly, but the houses depend on the Ascendant and Midheaven, which turn through the whole zodiac in about a day. Without the host's birth time (and birthplace) the houses cannot be drawn, so the guest's planets cannot be placed. The guest's birth time matters much less: only the guest's Moon, which moves 12° to 15° a day, is uncertain by more than a degree or two.

One house system for both charts. House systems divide the sky in different ways, and they can put the same degree in different houses, especially near a cusp. If each person's houses were drawn by a different system, the two directions of the overlay would be measured by different rules, and a difference between them could come from the method rather than the charts. Using one system for both places each person's planets in the other's houses in the same way.

Origin and history

Ptolemy compares the Ascendants ("horoscopes") of two charts, along with the Sun, Moon and Lot of Fortune, when judging friendship and enmity; agreement between the Ascendants he connects with friendship through pleasure or pain [1]. That is a comparison of angles, not a placing of one person's planets in the other's houses. Reading the guest's planets house by house in the host's chart belongs to modern synastry. Ronald Davison's Synastry (1977), for example, includes "house interchanges" among its methods of comparison, according to its publisher [3].

How it is used

Reading. Each guest planet is read by the nature of the planet and the meaning of the host's house: the guest's Saturn in the host's 2nd house is linked with the host's money and resources, the guest's Moon in the host's 4th with home and family, and so on. Planets on or near an angle of the host's chart (the 1st, 4th, 7th and 10th cusps) are often given the most weight.

Near a cusp. A planet within a degree or two of a cusp may be placed differently by another house system, or by a small error in the host's birth time. Such placements are best read as belonging to both houses, or checked against the host's time.

Unknown birth time. If the host's birth time is not known, overlays into that person's houses cannot be known either. The overlay the other way round is still possible, as long as the other person's time is known.

On this site. The House overlays tab of the Synastry tool shows two grids of twelve houses: the first person's planets in the second person's houses, and the second person's planets in the first person's houses. The planets from the Sun to Pluto and the Moon's true Node are placed. Each occupied house has an Interpret button. One house system, chosen on the form, is used for both charts (Placidus by default). If a person's birth time is unknown, that person's houses are not drawn and nothing is placed in them, since at another hour the houses would be entirely different; their own planets (taken at noon, with the Moon marked approximate) are still placed in the other person's houses.

Examples

Albert Einstein (14 March 1879, 11:30 local mean time, Ulm, 48°24′N 9°59′E; 10:50:02 UT) is the host. Mileva Marić (19 December 1875, Titel, about 45°12′N 20°18′E) is the guest. Her birth time is not recorded; Astro-Databank gives about 10 a.m., estimated from a narrative in a biography [2]. Her planets are taken at noon local mean time (10:38:48 UT).

Einstein's Placidus cusps: 1st 11°38′ Cancer, 2nd 28°37′ Cancer, 3rd 17°48′ Leo, 4th 12°50′ Virgo, 5th 18°19′ Libra, 6th 3°06′ Sagittarius, 7th 11°38′ Capricorn, 8th 28°37′ Capricorn, 9th 17°48′ Aquarius, 10th 12°50′ Pisces, 11th 18°19′ Aries, 12th 3°06′ Gemini.

• Her time barely matters here. During her birth day her Moon moved from 18°37′ Virgo to 1°26′ Libra, all within Einstein's 4th house (12°50′ Virgo to 18°19′ Libra). Every other planet also stays in the same house at any hour.
• Her Venus in his 7th, the house of marriage, 7°08′ past the cusp.
• The house system matters near cusps. In Koch houses Einstein's 3rd cusp is 22°54′ Leo, his 9th 22°54′ Aquarius and his 11th 8°20′ Taurus, so Mileva's Uranus moves to his 2nd house, her Saturn to his 8th and her Neptune to his 10th. Her Uranus is only 1°56′ past his Placidus 3rd cusp.
• The reverse is not possible. Einstein's planets in Mileva's houses would need her birth time. At noon her Ascendant would be 25°24′ Pisces, but in the course of that day it passed through every sign, so any house placement of his planets in her chart is a guess.