The same instant, two clock readings
Suppose you and a friend compare the Moon’s position. One uses Europe/London; the other uses America/New_York. At 16:00 UTC on 15 July 2024, their clocks read 17:00 and 12:00 respectively. Both calculations below return the same geocentric longitude because they sample the same instant.
| Time zone | Local time | UTC | Moon longitude |
|---|---|---|---|
| Europe/London | 17:00 (UTC+1) | 16:00 | 222.669069° |
| America/New_York | 12:00 (UTC−4) | 16:00 | 222.669069° |
Both are Scorpio 12° 40′ 09″ when rounded to the nearest arcsecond. The location names select clock rules, not a surface observer.
The same local noon, five hours apart
Now keep 12:00 as the daily sampling time and change only the time zone. On this date, London uses UTC+1 and New York UTC−4. The second calculation samples the Moon five hours later. The difference is about 2.54°, even though both local clocks say noon and both dates say 15 July.
| Time zone | Local time | UTC | Moon longitude |
|---|---|---|---|
| Europe/London | 12:00 (UTC+1) | 11:00 | 220.126488° |
| America/New_York | 12:00 (UTC−4) | 16:00 | 222.669069° |
At London noon the Moon is Scorpio 10° 07′ 35″; at New York noon it is Scorpio 12° 40′ 09″. Subtracting the unrounded longitudes gives 2.542581° to six decimal places.
What this example holds constant
These are fixed historical example results, calculated on 17 September 2026 with the Sky Degrees shipping Skyfield/JPL DE440 runtime. The requests use Calendar mode, July 2024, the tropical zodiac and the seven traditional bodies; the tables extract only 15 July and the Moon. Positions are geocentric. No birthplace, houses or personal birth record enters the calculation.
This is not a live sky table. The extra decimal places make the comparison reproducible; they do not promise equivalent physical accuracy. The five-hour clock difference applies to this date, not every date: London and New York change daylight-saving time on different schedules. Use the IANA zone names rather than assuming a permanent offset.
If two ephemerides still differ
Check the UTC date as well as the time, then the zodiac and the observer convention. A tropical longitude is not directly comparable with a sidereal longitude; a position measured from Earth’s centre is not the same calculation as one measured from an observer on its surface. The latter distinction matters especially for the nearby Moon.
Finally, compare the source and calculation method. Matching the clock removes one source of disagreement; it does not prove two engines implement every astronomical convention identically. A daily sample also cannot tell you the exact time a planet enters a sign or changes direction.
TRY IT YOURSELF
One small experiment.
- Open Daily ephemeris and choose July 2024. Expand Calculation settings, choose Tropical, enter Europe/London and 12:00, then Apply settings. Read the Moon on 15 July.
- Change only the time zone to America/New_York and apply. On 15 July, compare the Moon’s degree and the UTC moment in Calculation basis → Source and calculation record.
- Set Europe/London and 17:00, then apply. The 15 July Moon position should match New York at 12:00. These are two clock labels for 16:00 UTC.
No account or birth details needed. Set the month, time and zone using the steps above. Settings also update Current sky in this browser.
Sources & further reading
Definitions and software documentation explain methods. They do not establish the predictive validity of astrology.