Chasing planets isn't as silly as you might think, since afterall the earths spin can make a static body appear to move in relationship to itself.
I would suggest there is alot of sightings of "odd" things that can't be identified straight away, but I don't think that these would necessarily be alien craft since there are so many different theories to what a heavenly body might be.
As mentioned in many other threads some reported sightings are weather balloons, others are balloons that people concoct themselves, then theirs model airplanes and kites which people do still use, and even amatuer rocket enthusiasts.
Not to forget things like microlights and paragliders, the latter of course using thermal layers to gain altitude so they will tend to circle near a good thermal layer. (In fact the sleeve of which a person suits themselves into might be metalic to deal with some of the cold of altitude.)
The of course theres the obvious aircraft traffic, both civilian and Military (The later of which don't discuss their operations with civilian ground control which can cause problems as seen by many crashes involving military and civilian craft.)
There are then turbulance equations, where cloud spouts through differing pressure can generate abnormalities, which in turn could reflect the light differently than the rest of the sky. There is also the very small potential that the manipulation of particles through radiology within the atmosphere could generate alterations in weather behaviour as well as generating abnormalities that appear to be unidentified.
During the night you can see satellites on low orbital paths blinking as they move across the sky, and even some of the spacial platforms that were being put together in the event of an international space station. (Afterall Mir could be seen while it was still in orbit.) Theres of course all the space debris to that exists in orbit like dieing satellites.
Then there is meteor's that are madeup from the debris (both rock and space exploitary [and I do mean exploit]) that will streak across the skies shedding piece by piece as the atmosphere and it's force of decent causes it to break up.
It doesn't leave much to actually be "Alien Flying Craft", and you would still have to question the many why's and how's something would be flying in the sky. For instance
why would an alien craft "Hover", you might say "Observation" but satellites can act as observation platforms where supposedly a newspaper headline can be read on the ground (And even future technological advances look set to go further by identify people by their Biometric scent [Gene sniffing] or even being able to read a persons thought, perhaps even large instances of Gene therapy [Although this advance has alot of fears from eugenic use as undesirable elements are sequenced out.])
I suppose you can say I'm very much a skeptic of something in the sky being flown by an "Alien", or in fact whether it is a UFO, since there is little left to be "Unidentified".
