Birth Chart Calculator Accuracy: Tests & Limitations
Is your chart right? We checked our calculator against NASA/JPL data for ten charts — three famous birth records and seven test dates from 1900 to 2100 — and publish every result, including the limits of the test.
Across 100 planet comparisons, the largest difference from NASA/JPL was 0.022° (1.33′). That is far smaller than anything that would normally change a sign or a degree in your chart.
These examples use the Western tropical zodiac.
What these examples show
- Planet comparisons
- 100
- Largest difference
- 0.0221° (1.33′)
- Average difference
- 0.0014° (0.09′)
- Differences above 1°
- 0
References captured:
Why your chart may differ from another site
Tropical or sidereal (Vedic) zodiac
Coeluna uses the Western tropical zodiac. Vedic and other sidereal charts measure from the stars instead, which today shifts every position back by about 24°, so many planets land in the previous sign. Both can be calculated correctly; they simply start from different points.
House system
We use Placidus houses, switching to equal houses near the poles. Whole Sign, Koch or Equal houses can place a planet in a different house even when every position agrees.
Birth time and time zone
A wrong time zone, a missed daylight-saving change or a rounded birth time is the most common reason for a different Rising sign. The Rising degree moves about one degree every four minutes.
Lunar node
Coeluna shows the mean lunar node. Sites that use the true node can differ by more than a degree, and occasionally by a sign.
Compared with an independent reference
Each table compares the Sun, Moon and eight planets with positions from NASA/JPL Horizons, the system used to plan space missions. Every example stays in the report, and any difference above 1° would be flagged.
Marilyn MonroePublished birth record
- Local birth time
- 1926-06-01 · 09:30
- Universal Time
- 1926-06-01 17:30
| Planet | Coeluna | NASA/JPL | Difference |
|---|---|---|---|
| Sun | Gemini 10°26′39″ | Gemini 10°26′40″ | 0.1″ |
| Moon | Aquarius 19°06′12″ | Aquarius 19°05′54″ | 17.5″ |
| Mercury | Gemini 6°46′43″ | Gemini 6°46′43″ | 0.1″ |
| Venus | Aries 28°45′01″ | Aries 28°45′01″ | 0.1″ |
| Mars | Pisces 20°43′53″ | Pisces 20°43′54″ | 0.1″ |
| Jupiter | Aquarius 26°49′32″ | Aquarius 26°49′32″ | 0.0″ |
| Saturn | Scorpio 21°26′30″ | Scorpio 21°26′30″ | 0.0″ |
| Uranus | Pisces 28°59′41″ | Pisces 28°59′41″ | 0.0″ |
| Neptune | Leo 22°13′22″ | Leo 22°13′22″ | 0.1″ |
| Pluto | Cancer 13°24′03″ | Cancer 13°23′30″ | 33.9″ |
Rising / Ascendant · Coeluna: Leo 13°04′06″ · Published birth record: Leo 13°04′00″ · Difference: 5.6″
The published Rising values are rounded to one arcminute. No independently sourced Midheaven or house-cusp values are claimed here. Source
Barack ObamaPublished birth record
- Local birth time
- 1961-08-04 · 19:24
- Universal Time
- 1961-08-05 05:24
| Planet | Coeluna | NASA/JPL | Difference |
|---|---|---|---|
| Sun | Leo 12°32′52″ | Leo 12°32′52″ | 0.1″ |
| Moon | Gemini 3°21′39″ | Gemini 3°21′27″ | 12.1″ |
| Mercury | Leo 2°19′54″ | Leo 2°19′54″ | 0.0″ |
| Venus | Cancer 1°47′22″ | Cancer 1°47′22″ | 0.1″ |
| Mars | Virgo 22°34′36″ | Virgo 22°34′36″ | 0.1″ |
| Jupiter | Aquarius 0°51′31″ | Aquarius 0°51′31″ | 0.1″ |
| Saturn | Capricorn 25°19′51″ | Capricorn 25°19′51″ | 0.1″ |
| Uranus | Leo 25°16′15″ | Leo 25°16′15″ | 0.1″ |
| Neptune | Scorpio 8°36′21″ | Scorpio 8°36′21″ | 0.0″ |
| Pluto | Virgo 6°59′09″ | Virgo 6°58′40″ | 29.1″ |
Rising / Ascendant · Coeluna: Aquarius 18°02′41″ · Published birth record: Aquarius 18°03′00″ · Difference: 19.4″
The published Rising values are rounded to one arcminute. No independently sourced Midheaven or house-cusp values are claimed here. Source
Elizabeth IIPublished birth record
- Local birth time
- 1926-04-21 · 02:40
- Universal Time
- 1926-04-21 01:40
| Planet | Coeluna | NASA/JPL | Difference |
|---|---|---|---|
| Sun | Taurus 0°12′21″ | Taurus 0°12′21″ | 0.1″ |
| Moon | Leo 12°07′39″ | Leo 12°07′18″ | 20.4″ |
| Mercury | Aries 4°39′43″ | Aries 4°39′43″ | 0.0″ |
| Venus | Pisces 13°57′22″ | Pisces 13°57′22″ | 0.2″ |
| Mars | Aquarius 20°51′58″ | Aquarius 20°51′58″ | 0.2″ |
| Jupiter | Aquarius 22°30′35″ | Aquarius 22°30′35″ | 0.0″ |
| Saturn | Scorpio 24°26′42″ | Scorpio 24°26′42″ | 0.0″ |
| Uranus | Pisces 27°21′21″ | Pisces 27°21′21″ | 0.0″ |
| Neptune | Leo 22°02′05″ | Leo 22°02′05″ | 0.1″ |
| Pluto | Cancer 12°42′45″ | Cancer 12°42′22″ | 22.8″ |
Rising / Ascendant · Coeluna: Capricorn 21°23′59″ · Published birth record: Capricorn 21°24′00″ · Difference: 0.5″
The published Rising values are rounded to one arcminute. No independently sourced Midheaven or house-cusp values are claimed here. Source
Greenwich · 1900Controlled example
- Local birth time
- 1900-01-01 · 12:00
- Universal Time
- 1900-01-01 12:00
| Planet | Coeluna | NASA/JPL | Difference |
|---|---|---|---|
| Sun | Capricorn 10°39′48″ | Capricorn 10°39′48″ | 0.0″ |
| Moon | Capricorn 9°36′42″ | Capricorn 9°36′59″ | 16.4″ |
| Mercury | Sagittarius 19°38′21″ | Sagittarius 19°38′21″ | 0.1″ |
| Venus | Aquarius 6°59′46″ | Aquarius 6°59′46″ | 0.1″ |
| Mars | Capricorn 14°15′11″ | Capricorn 14°15′11″ | 0.2″ |
| Jupiter | Sagittarius 1°14′01″ | Sagittarius 1°14′01″ | 0.1″ |
| Saturn | Sagittarius 27°46′30″ | Sagittarius 27°46′30″ | 0.0″ |
| Uranus | Sagittarius 10°10′01″ | Sagittarius 10°10′00″ | 0.1″ |
| Neptune | Gemini 25°12′18″ | Gemini 25°12′18″ | 0.3″ |
| Pluto | Gemini 15°13′57″ | Gemini 15°14′34″ | 37.1″ |
Greenwich · 2000Controlled example
- Local birth time
- 2000-01-01 · 12:00
- Universal Time
- 2000-01-01 12:00
| Planet | Coeluna | NASA/JPL | Difference |
|---|---|---|---|
| Sun | Capricorn 10°22′08″ | Capricorn 10°22′08″ | 0.0″ |
| Moon | Scorpio 13°19′39″ | Scorpio 13°19′26″ | 13.7″ |
| Mercury | Capricorn 1°53′21″ | Capricorn 1°53′21″ | 0.1″ |
| Venus | Sagittarius 1°33′57″ | Sagittarius 1°33′57″ | 0.1″ |
| Mars | Aquarius 27°57′48″ | Aquarius 27°57′48″ | 0.1″ |
| Jupiter | Aries 25°15′11″ | Aries 25°15′11″ | 0.2″ |
| Saturn | Taurus 10°23′44″ | Taurus 10°23′44″ | 0.1″ |
| Uranus | Aquarius 14°48′33″ | Aquarius 14°48′33″ | 0.3″ |
| Neptune | Aquarius 3°11′35″ | Aquarius 3°11′35″ | 0.6″ |
| Pluto | Sagittarius 11°27′50″ | Sagittarius 11°27′17″ | 32.9″ |
Greenwich · 2100Controlled example
- Local birth time
- 2100-12-31 · 12:00
- Universal Time
- 2100-12-31 12:00
| Planet | Coeluna | NASA/JPL | Difference |
|---|---|---|---|
| Sun | Capricorn 9°51′21″ | Capricorn 9°51′15″ | 5.9″ |
| Moon | Capricorn 16°42′49″ | Capricorn 16°41′29″ | 1.33′ |
| Mercury | Capricorn 27°35′29″ | Capricorn 27°35′21″ | 8.2″ |
| Venus | Sagittarius 25°11′29″ | Sagittarius 25°11′21″ | 7.2″ |
| Mars | Sagittarius 13°39′08″ | Sagittarius 13°39′04″ | 4.1″ |
| Jupiter | Scorpio 16°55′22″ | Scorpio 16°55′21″ | 1.3″ |
| Saturn | Scorpio 6°55′05″ | Scorpio 6°55′04″ | 1.0″ |
| Uranus | Aries 21°43′53″ | Aries 21°43′53″ | 0.3″ |
| Neptune | Virgo 19°30′23″ | Virgo 19°30′21″ | 2.3″ |
| Pluto | Taurus 3°22′37″ | Taurus 3°22′54″ | 16.7″ |
Sydney · 2000Controlled example
- Local birth time
- 2000-01-01 · 12:00
- Universal Time
- 2000-01-01 01:00
| Planet | Coeluna | NASA/JPL | Difference |
|---|---|---|---|
| Sun | Capricorn 9°54′06″ | Capricorn 9°54′06″ | 0.0″ |
| Moon | Scorpio 7°48′05″ | Scorpio 7°47′51″ | 14.1″ |
| Mercury | Capricorn 1°10′36″ | Capricorn 1°10′36″ | 0.0″ |
| Venus | Sagittarius 1°00′42″ | Sagittarius 1°00′42″ | 0.1″ |
| Mars | Aquarius 27°36′28″ | Aquarius 27°36′28″ | 0.1″ |
| Jupiter | Aries 25°14′05″ | Aries 25°14′05″ | 0.2″ |
| Saturn | Taurus 10°24′18″ | Taurus 10°24′18″ | 0.1″ |
| Uranus | Aquarius 14°47′10″ | Aquarius 14°47′10″ | 0.3″ |
| Neptune | Aquarius 3°10′37″ | Aquarius 3°10′36″ | 0.6″ |
| Pluto | Sagittarius 11°26′52″ | Sagittarius 11°26′19″ | 33.0″ |
Tromsø · 2000Controlled example
- Local birth time
- 2000-01-01 · 12:00
- Universal Time
- 2000-01-01 11:00
| Planet | Coeluna | NASA/JPL | Difference |
|---|---|---|---|
| Sun | Capricorn 10°19′35″ | Capricorn 10°19′35″ | 0.0″ |
| Moon | Scorpio 12°49′36″ | Scorpio 12°49′22″ | 13.7″ |
| Mercury | Capricorn 1°49′28″ | Capricorn 1°49′28″ | 0.1″ |
| Venus | Sagittarius 1°30′56″ | Sagittarius 1°30′55″ | 0.1″ |
| Mars | Aquarius 27°55′52″ | Aquarius 27°55′52″ | 0.1″ |
| Jupiter | Aries 25°15′05″ | Aries 25°15′05″ | 0.2″ |
| Saturn | Taurus 10°23′47″ | Taurus 10°23′47″ | 0.1″ |
| Uranus | Aquarius 14°48′26″ | Aquarius 14°48′25″ | 0.3″ |
| Neptune | Aquarius 3°11′30″ | Aquarius 3°11′29″ | 0.6″ |
| Pluto | Sagittarius 11°27′45″ | Sagittarius 11°27′12″ | 33.0″ |
New York · winter 2024Controlled example
- Local birth time
- 2024-01-15 · 12:00
- Universal Time
- 2024-01-15 17:00
| Planet | Coeluna | NASA/JPL | Difference |
|---|---|---|---|
| Sun | Capricorn 25°01′51″ | Capricorn 25°01′51″ | 0.0″ |
| Moon | Pisces 22°54′52″ | Pisces 22°54′45″ | 7.2″ |
| Mercury | Capricorn 1°46′18″ | Capricorn 1°46′18″ | 0.0″ |
| Venus | Sagittarius 20°35′17″ | Sagittarius 20°35′17″ | 0.1″ |
| Mars | Capricorn 8°17′36″ | Capricorn 8°17′36″ | 0.2″ |
| Jupiter | Taurus 5°59′58″ | Taurus 5°59′58″ | 0.3″ |
| Saturn | Pisces 4°39′32″ | Pisces 4°39′31″ | 0.3″ |
| Uranus | Taurus 19°08′48″ | Taurus 19°08′49″ | 1.2″ |
| Neptune | Pisces 25°20′53″ | Pisces 25°20′51″ | 1.6″ |
| Pluto | Capricorn 29°50′02″ | Capricorn 29°49′36″ | 25.7″ |
New York · summer 2024Controlled example
- Local birth time
- 2024-07-15 · 12:00
- Universal Time
- 2024-07-15 16:00
| Planet | Coeluna | NASA/JPL | Difference |
|---|---|---|---|
| Sun | Cancer 23°39′00″ | Cancer 23°39′00″ | 0.1″ |
| Moon | Scorpio 12°40′08″ | Scorpio 12°40′09″ | 0.5″ |
| Mercury | Leo 19°35′43″ | Leo 19°35′43″ | 0.0″ |
| Venus | Leo 4°53′57″ | Leo 4°53′57″ | 0.1″ |
| Mars | Taurus 26°22′49″ | Taurus 26°22′49″ | 0.0″ |
| Jupiter | Gemini 11°17′19″ | Gemini 11°17′19″ | 0.3″ |
| Saturn | Pisces 19°13′12″ | Pisces 19°13′12″ | 0.3″ |
| Uranus | Taurus 26°19′36″ | Taurus 26°19′38″ | 1.2″ |
| Neptune | Pisces 29°53′06″ | Pisces 29°53′04″ | 1.6″ |
| Pluto | Aquarius 1°02′38″ | Aquarius 1°02′55″ | 17.4″ |
What this evidence cannot tell you
Ten examples are a sample, not a guarantee for every date between 1900 and 2100.
Agreement on astronomical positions says nothing about astrological interpretations or predictions.
Reference sources apply slightly different corrections, especially for the Moon, Pluto and dates far from today, when the exact rotation of the Earth has to be estimated.
An accurate calculation still needs accurate birth details. An uncertain time can change your Rising sign, houses and Moon-dependent readings.
For the technically curious
Does the smaller planetary table lose precision?
We also compare our shipped planetary tables with their full versions at the same moments. This isolates the effect of trimming: both calculations share the same methods. It does not independently test those methods. Earth contributes to every comparison; the Moon, Pluto, lunar node and houses are outside this check.
Largest trimming difference · 0.0000561° (0.00337′)
Full tables / Shipped tables
Marilyn Monroe
- Sun: 0.0000251°
- Mercury: 0.0000067°
- Venus: 0.0000214°
- Mars: 0.0000189°
- Jupiter: 0.0000038°
- Saturn: 0.0000150°
- Uranus: 0.0000104°
- Neptune: 0.0000047°
Barack Obama
- Sun: 0.0000105°
- Mercury: 0.0000042°
- Venus: 0.0000082°
- Mars: 0.0000170°
- Jupiter: 0.0000130°
- Saturn: 0.0000142°
- Uranus: 0.0000164°
- Neptune: 0.0000054°
Elizabeth II
- Sun: 0.0000069°
- Mercury: 0.0000007°
- Venus: 0.0000327°
- Mars: 0.0000435°
- Jupiter: 0.0000038°
- Saturn: 0.0000125°
- Uranus: 0.0000125°
- Neptune: 0.0000066°
Greenwich · 1900
- Sun: 0.0000109°
- Mercury: 0.0000014°
- Venus: 0.0000050°
- Mars: 0.0000058°
- Jupiter: 0.0000063°
- Saturn: 0.0000007°
- Uranus: 0.0000446°
- Neptune: 0.0000002°
Greenwich · 2000
- Sun: 0.0000013°
- Mercury: 0.0000036°
- Venus: 0.0000083°
- Mars: 0.0000092°
- Jupiter: 0.0000064°
- Saturn: 0.0000000°
- Uranus: 0.0000064°
- Neptune: 0.0000004°
Greenwich · 2100
- Sun: 0.0000105°
- Mercury: 0.0000210°
- Venus: 0.0000005°
- Mars: 0.0000055°
- Jupiter: 0.0000155°
- Saturn: 0.0000307°
- Uranus: 0.0000111°
- Neptune: 0.0000288°
Sydney · 2000
- Sun: 0.0000001°
- Mercury: 0.0000028°
- Venus: 0.0000079°
- Mars: 0.0000084°
- Jupiter: 0.0000066°
- Saturn: 0.0000000°
- Uranus: 0.0000063°
- Neptune: 0.0000004°
Tromsø · 2000
- Sun: 0.0000012°
- Mercury: 0.0000035°
- Venus: 0.0000082°
- Mars: 0.0000092°
- Jupiter: 0.0000065°
- Saturn: 0.0000000°
- Uranus: 0.0000064°
- Neptune: 0.0000004°
New York · winter 2024
- Sun: 0.0000246°
- Mercury: 0.0000239°
- Venus: 0.0000015°
- Mars: 0.0000222°
- Jupiter: 0.0000092°
- Saturn: 0.0000064°
- Uranus: 0.0000561°
- Neptune: 0.0000477°
New York · summer 2024
- Sun: 0.0000041°
- Mercury: 0.0000137°
- Venus: 0.0000053°
- Mars: 0.0000179°
- Jupiter: 0.0000159°
- Saturn: 0.0000175°
- Uranus: 0.0000541°
- Neptune: 0.0000446°
Time conversion and chart geometry
Separate checks compare local times with recorded Universal Time, verify twelve opposite house cusps and preserve the real Midheaven. Tromsø uses equal houses because our calculator switches systems at polar latitudes. These are consistency checks, not independent measurements of every house or the lunar node.
10 of 10 examples pass every consistency check: local time to Universal Time, twelve opposite house cusps, Rising and Midheaven alignment, and the lunar node.
Reference precision: 0.0000001° · NASA/JPL Horizons