Moon Phase Without a Birth Time: Checking the Whole Birth Date

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If you do not know your birth time, your natal Moon phase is often still knowable. On most dates the Moon stays in one phase from midnight to midnight. On some it crosses into the next phase partway through the day, and then the phase depends on a time you do not have. This page shows how to tell which kind of date yours is, using two runs of the free chart, and what to do when the answer is "it depends". The phases themselves are defined on natal Moon phases explained.

Calculator note: The Astrologist.com birth chart does not calculate or label Moon phases. It shows the Sun and Moon by sign, degree and minute, and the phase described here is worked out by hand from those two positions. Without a birth time, the chart places the Moon at local noon as a date reference only, so a phase read from that position is not established until the whole birth date has been checked.

Why the noon Moon is not enough

When you leave the birth time empty, the free chart calculates everything at local noon as an internal reference. It marks the Moon with ≈, notes that its position is a date reference and not your natal longitude, and leaves out the Ascendant, the houses and every Moon aspect line. It also compares the Moon's sign at 00:00 and 23:59 and lists both signs if they differ.

That sign check does not cover the phase. The phase depends on the Moon's distance from the Sun, and in a single day the Moon gains on the Sun by roughly a quarter to a third of a phase's width. In calculations with the free chart's routines for London, every date from 1900 to 2099, the directed elongation (Moon minus Sun) grew by 10.3° to 14.8° between 00:00 and 23:59, 12.2° on average. A phase is 45° wide, so on 19,778 of the 73,049 dates, 27.1%, the phase changed during the local day. New York gave the same share. Because no single day ever moved the Moon more than 15° relative to the Sun, a date can hold at most two phases, never three.

The full-day check

1. Run the free birth chart for your birth date and place with the time set to 00:00.
2. Run it again with the time set to 23:59.
3. For each run, take the Sun and Moon from the planet table and work out the elongation as described in how to find your natal Moon phase.
4. Ignore the Rising sign, houses and aspect lines of these two test charts. They belong to the test times, not to you.

If both values fall in the same phase, that phase holds for the whole date. This works because the Moon always moves forward faster than the Sun, so the elongation keeps increasing through the day when it is counted continuously, without resetting at 360°, and every value in between lies between the two you calculated. Unlike the endpoint check for a slow planet near a station, there is no doubling back to worry about.

One adjustment is needed when the day crosses the New Moon. If the 23:59 value is smaller than the 00:00 value, the elongation passed 360° and started again from 0°. Add 360° to the 23:59 value before comparing: 359.6° at 00:00 and 10.9° at 23:59 become 359.6° and 370.9°. Every elongation during that day lies between those two numbers, and the only boundary inside the interval is 360°, the New Moon. Then subtract 360° from any value above 360° to read its phase: 370.9° is 10.9°, New. The 22 April 1993 example below works exactly this way.

If the two values fall in different phases, you have two possible phases, and the change happened at a definite moment that day.

On a few dates in some places the clocks changed at midnight, so 00:00 did not exist and the chart will reject it. Use the first time of day the chart accepts instead.

Three dates, three outcomes

All positions are from the free chart's routines for London; local times are in the clock time of the day.

8 April 1987: settled. At 00:00 the elongation was 107.9°, at 23:59 it was 118.9°. Both are First Quarter, so anyone born in London that day has a First Quarter Moon. The Sun-Moon trine is a different story: it came into orb at 112° at about 09:01, so without a time the trine stays open even though the phase does not.

6 April 1987: two possible phases. At 00:00 the Sun was at 15°37' Aries and the Moon at 11°38' Cancer, an elongation of 86.0°: Crescent. At 23:59 it was 96.9°: First Quarter. The exact First Quarter Moon fell at about 08:48 London time (BST). The Moon was in Cancer all day, so the free chart reports a certain Moon sign, and its noon reference gives 91.5°, which would point to First Quarter. For anyone born before about 08:48, that would be the wrong phase.

22 April 1993: a change just after midnight. At 00:00 the Sun was at 1°50' Taurus and the Moon at 1°27' Taurus, 359.6°: Balsamic, only 0.4° short of the New Moon. The New Moon came at about 00:51 (BST), and from then on the phase was New, reaching 10.9° by 23:59. A birth in the first 51 minutes of that date is Balsamic; any later birth is New.

When an endpoint sits close to a boundary

The planet table shows whole minutes of arc, so an elongation worked out from it can be off by up to about a minute and a half, roughly three minutes of clock time. In the London sample, one or both ends of the day were within 0.1° of a boundary on 599 dates (0.8%). If either of your values is that close, or if you need the moment of the change to compare with a rough time, use a finer source. When the boundary is 0°, 90°, 180° or 270°, an ephemeris or a lunar phase table gives the time of the New Moon, First Quarter, Full Moon or Last Quarter to the minute. Convert that time to your local clock time, including any daylight saving in force, and compare it with what you know about your birth. Phase tables do not list the 45°, 135°, 225° and 315° boundaries. For those you need Sun and Moon longitudes more precise than whole minutes of arc, or an ephemeris calculation of the moment the elongation reached that value; do not infer the crossing from the nearest New, quarter or Full Moon, or by assuming each phase lasts the same number of days. You can also narrow the moment by running the free chart at times in between, keeping in mind the same rounding. If the precision you have cannot settle it, keep both phases open.

If two phases remain

• Use any time evidence you have. "Early morning" or "after lunch" is often enough once you know the change time. How to find your birth time lists sources worth checking.
• Read both phases. Neighbouring phases share a half of the cycle, except across the Full and New Moons, and a reading can hold both without choosing. Born under a waxing Moon and born under a waning Moon cover all eight.
• Do not choose by preference. Picking the phase whose description sounds better is not evidence for a birth time. Astrologers who estimate missing times use life events instead, with the limits explained on birth time rectification.

The same full-day approach is used for the Moon's sign on Moon sign without a birth time, and the wider picture of an unknown-time chart is on what your chart can still show. For a chart you are reading by hand, you can also ask the Astrologist to talk through both possible phases.

Related articles

• How to Find Your Natal Moon Phase
• Moon Phase vs Sun-Moon Aspects
• Can You Know Your Moon Sign Without a Birth Time?
• How to Read a Chart Without a Birth Time