Have you ever tried to look up the exact time of a Full or New Moon, or the start of Mercury Retrograde, only to find the timing to vary by a day or two, depending on the source you consult?
I've wondered about that too. Could my computer generated ephemeris be faulty, I've asked. What system of calculation are others using, I've wondered. This became a puzzle I had to solve.
The most obvious answer is that various time zones are used when reporting astrological events. Many writers use local times, which makes a lot of sense in assisting readers who are not usually interested in the more technical aspects of astrological calculations. But not all writers are careful about noting which time zone they are using, and then others elsewhere in the world repeat that time without realizing the reference point.
Others use something called Coordinated Universal Time (abbreviated confusingly as UTC). Technically, UTC is not a time zone, but for part of the year UTC aligns exactly with Greenwich (England) Mean Time (GMT) which is where time zones start. Things get a bit more confusing in summer, though, when Britain goes on daylight savings and switches to British Summer Time (BST), which is an hour off from UTC).
So if novice astrologer doesn't know that UTC and GMT aren't always the same, this can create a difference in reported time as well. And to make matters worse, England doesn't go to BST on the same dates that the US switches to daylight savings. So there's another potential for disjointed timing of astrological events.
The Astrological Ephemeris
An ephemeris is a chart of the planetary positions by degree, sign, and minute, (and by second if super exact precision is required). So in looking at the ephemeris set to the minute for this current / pending Mercury Retrograde what we see at left is that Mercury is quite stationary at 23 degrees and 36 minutes of Taurus before and after the time of 5:20 pm Greenwich Mean Time when the designation of retrograde is given.
But retrograde means "moving backwards", and clearly Mercury isn't appearing to move at all yet. Instead, it is in a status known as "stationing", and because the change is from direct motion to retrograde motion, we say it is stationing retrograde. It's not technically retrograde yet.
By the way, at the end of a retrograde the same thing happens -- Mercury will appear to be motionless for a while before returning to forward movement, and we called that "stationing direct."
While a planet is in a stationing status, it does have small and very slow movement. Some astrologers consider any movement as indicative of being out of stationing and in to the retro or direct period. Others who are concerned with precise timing, wait until the planet picks up its normal rate of speed before declaring it to be out of stationing status.
This waiting period can be as much as a day or two, and is always several hours.
Ephemeris vs Animated Charts
A last reason that the time for astrological events can be different from one report to another is that if an astrologer is looking at chart wheels rather than an ephemeris. Advancing the time with a software feature of animating the chart itself show a change in symbol from the S for stationing to the Rx for retrograde (or to D for direct). The chart's software is likely to take the conservative approach noted above and display the switch at the moment that the planet has achieved its normal rate of speed.
And that is probably 2 tons and a bushel more than you ever wanted to know about why different sources claim different times for the start and finish of different astrological events.