An example of a historical idea that would now be considered an alternative conception,
"Comets were supposed to originate from matter ejected radially outwards (at constant speed) form an axially rotating celestial body which tends to condense after doming into contact with the latter's circulating atmosphere. This condensation acquires an ever-increasing velocity on account of the augmentation in the circular motion at greater distances from the centre of the parent body. The net effect of the two components is to produce a 'spin off' whereby the condensation – or comet, as it now becomes – flies off tangentially and continues along a straight line, always facing the Sun since it is largely composed of a similar substance.
…By claiming that a comet seen moving towards the Sun in November was the same as one moving in the opposite direction a month or so later, Flamsteed was rejecting the currently held view of Kepler, Hevelius, and others that comets moved in straight lines."
Eric G. Forbes (1975) Introduction, in The Gresham Lectures of John Flamsteed, London: Mansell Information Publishing Ltd.
Forbes is reporting on a proposal published in the Philosophical Transactions in 1668. (The suggestions that the comet would face the sun because of its similar composition appears to reflect Aristotelian ideas.)
