Find and identify 3 alias periods for the variable at x=131 , y=202. Identify the periods (in terms of days) and the interpretation (one day alias, correct period, twice one days alias or twice correct period)
Aliasing is a phenomenon in astronomical observation in which a cycle is observed in periods that are too short, or infrequent, so that a false reading of cycle frequency or of target amplitude is attained. Because observations of astronomical targets are initiated and subsequently continued at times that are arbitrary to the cycle of the target (target cycles don't conveniently coincide with astronomers' waking, sleeping, working and eating schedules), until a full cycle is observed, it can't be known at what point in the cycle the target is being observed.
Illustrate that with observing 1 cycle of 4 seconds, with each of 4 movements in the cycle taking 1 second each: (1 cycle)/(4 seconds) = .25 Hz. Depending upon what phase of the cycle your observation falls in, instead of measuring the variability as (1 cycle)/(4 seconds) = .25 Hz, your measurements might be so far off as to result in a frequency of (1 cycle)/(20 seconds) = .05 Hz (Gregory Ogin, Physics Undergrad, St. Paul, MN, PhysLink.com). Your incorrect measurement would be the result of aliasing.
Aliasing can be avoided by measuring with great enough frequency. The rule of thumb is to measure 100 times more often than the highest expected frequency of cycle repetitions (e.g., ball bounces will have a lower expected frequency than the high frequency of star variability).