# When will solar eclipses become lunar transits?

When will the moon be far enough away from the Earth that we would no longer consider it a 'solar eclipse', but rather a 'lunar transit' when the moon travels between the Earth and Sun?

• I bet there's an online web page that will let you calculate the date of this occurrence. Feb 23 '17 at 13:59
• Every annular solar eclipse is essentially a lunar transit. Full solar eclipses are occultations.
– SF.
Feb 23 '17 at 14:18
• @CarlWitthoft: Yes, I assume there probably is. and with the information SF provided, looks like it's already happening but it's easier to always just commonly refer to it as an eclipse. When looking around I mostly just found stuff about solar eclipses and transits of Venus and Mercury. I wouldn't have asked here if google would have found it. Feb 23 '17 at 19:01

When looking at definitions of [transit] and [eclipse] or more general [occultations] it is noteworthy that the degree of covering is not defined, so technically speaking each eclipse is also a transit and an annular eclipse even more so.

Details for next eclipses can be found at [this NASA page]. The next annular eclipse, err, transit happens next Sunday 2017-02-26 and is visible in South America, south Atlantic and Afrika.

As you were referring to the [increasing distance between earth and moon] (+3.8 cm /yr), it then depends on an exact definition on when an eclipse is not regarded as an eclipse anymore. Assume for simplicity that this is the case, when the diameter of the moon has halved (one quarter the area), this means that the distance had to be doubled. This would take [380 Gm] / 3.8 cm/yr = 10 Tyr (using SI prefixes).

Note that one might need to take other effects into account, too, for example the [increasing distance of earth and sun] (~15 cm / a) and that the [sun is becoming a red giant] in a couple of Gyr.

Sorry, would have loved to post more links, but are not allowed to currently.

• No worries about the links. This is an awesome answer! Thank you so much! There's plenty of information for me to think about and look up, especially the sun getting larger and the earth moving away from it. I didn't consider that. It's also interesting that there isn't a specific point where the definition changes. It's more like "eclipse" is "transit+". Thanks again! Feb 24 '17 at 17:52
• That 10 trillion year figure really isn't valid. The Sun will be a white dwarf long before then, and the Earth-Moon system will have suffered major disruption during the Sun's red giant phase, and will most likely get swallowed at some stage, or at best have its solar orbit drastically modified because of the changing solar mass & friction with the huge amount of gas & dust that red giants spew out. Jan 24 '19 at 7:13
• But even if that weren't the case, it'd still be wrong. The 3.8cm/yr recession speed is only approximately constant, and IIRC the Earth would become tidally locked to the Moon in about 50 billion years (ignoring the red giant stuff) so recession would cease. Jan 24 '19 at 7:13