Several planets are assembling toward a rare alignment later this month, when five of them will crowd into a patch of sky small enough that all will be visible in a single glance. The setup will provide a planet-watching opportunity that won’t be repeated for a century. Even now, Jupiter, Mars and Saturn form a nearly straight line in the west each night. By late April, Mercury, Venus, Mars, and Saturn will all bunch up in the western sky just after sunset, with bright Jupiter also nearby. Three of the planets — Venus, Saturn and Mars — will crowd into an even smaller patch of sky in early May.