Just 0.6° apart, the close call of two planets just after sunset this month doesn't count as a conjunction for many astronomers. Continue reading →
¡SkyCaramba! Weekly astronomy blog for the week ending January 10, 2015
A casual definition of conjunction says it’s when two planets appear to be closest together in our sky. But the professional definition that astronomers use says one object must be due north of the other. They aren’t necessarily closest together at that time. And they may also appear very close together without being in conjunction. Mercury and Venus provide an excellent example for us this month.
Any line going from the north celestial pole to the south celestial pole is called a line of right ascension. Right ascension is measured all the way around the celestial sphere either in degrees from 0 to 360 or in hours from 0 to 24. Degrees and hours are each divided into minutes and seconds. However, they are identical in name and not in measure. When two planets are on the same line of right ascension, they are in conjunction in right ascension.
Some astronomers don’t like to acknowledge conjunctions in right ascension. Using that system of marking ...