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You got me curious, and I thought the Wikipedia article would have the answer, but instead I'm copying this from the talk page [wikipedia.org] on microlensing by somebody who claims to have worked as an astronomer:

I've tried to fix the fuzzyness of the definition of microlensing with a new one: Microlensing is the subset of gravitational lensing whose variations in time can be measured. Typically, this means that the lens mass must be small enough that it will cross its own Einstein ring [wikipedia.org] radius in less than the time it takes for a graduate student to finish a PhD thesis. The EROS collaboration is analysing our own and MACHOs data for evidence of very long time scale events of order 100s or 1000s of years (see sec. 5.6 of [1] [arxiv.org]). Although these may not be confirmed as microlensing events (rather than some other very long time-scale variability), the non-detection of these events would allow a limit on dark matter by 100-1000 Mo MACHOs. Any event where the lens mass is so big and far away that it takes millions of years to cross its einstein ring radius, and thus changes too slowly in time to be studied in the time domain, is a "macrolens".

I also disagree with the four definitions by MDAstronomer. It is not true that any lensing event with unresolved images is microlensing. A galaxy can lens a quasar, but have the images be too close to be resolved. This is not microlensing. Likewise,lensing by a compact object does not describe microlensing. "Any" gravitational lens must be physically smaller or about the same size as its own einstein radius to cause any measurable lensing effect. That is why we do not see any lensing effects from the Moon. It is so close that its Einstein radius is tiny. However, if the moon were a few kiloparsecs away, its einstein radius would be larger than its physical radius and it could be a perfectly ordinary microlens. The supermassive black hole at the center of the milky way is a compact object, but it cannot be a microlens... its einstein radius is too big for any changes in lensing to be monitored in time. It could in principle be a macrolens if there were a quasar right behind it. The microlensing at the edges of gravitationally lensed quasars is called microlensing because it causes time-varying effects in the apparent flux of the images. This has been significant because it interferes with attempts to use these gravitational lenses to measure the Hubble Constant.

The time-varying nature of a microlens is the key to all of its observations. And the need to take over large blocks of telescope time to do microlensing has revolutionized time-domain astronomy in general, in part through a bureaucratic reorganization of Telescope Allocation Committees and the advent of dedicated telescopes. There have been great resulting changes not only in microlensing but in searches for supernovae, asteroids, variable stars.

None of the various types of microlensing observations (photometric brightenning, astrometric shift, interferometric visibility reduction due to image splitting, shifts in color, spectrum, or variability amplitude) are strong enough to determine a microlensing event from a single observation. All of them require detecting some change in time, if only because there are plenty of natural causes that can mimic any one of the shifts for a single measurement. For example, how could one seperate a star which was split into two images from an ordinary binary star without time-domain information?

I disagree with Mike's splitting of lensing into strong, weak and microlensing. A lens is strong if it is within one Einstein radius of the line of sight to the source, and weak otherwise. Nearly all photometric microlenses are strong lenses, but astrometric lensing is much more sensitive to weak lensing than photometric lensing. Eddington's verification of general relativity was classic weak astrometric microlensing, and we will use SIM to study weak astrometric microlensing to measure the masses of other stars [2] [harvard.edu]. David s graff [wikipedia.org] 22:17, 2 June 2006 (UTC)

Source: http://rss.slashdot.org/~r/Slashdot/slashdotScience/~3/tlmiHe4wIoQ/story01.htm

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