Lunar occultation and graze

BY MARK ARMSTRONG
ASTRONOMY NOW

 

The occulation as seen from London at 9:30pm BST on April 25.

         On the evening of 25 April a 4.55 day-old crescent moon moves in front of the moderately bright star zeta Tauri (magnitude +2.9). From Greenwich the star is occulted at 9.30pm BST in the deepening twilight with the Moon quite well placed 25 degrees above the western horizon. Fortunately the occultation occurs at the dark lunar limb as such disappearances are much easier to see than those that happen at the bright limb. This can be seen in this event when zeta Tauri reappears at the bright cusp at around 9.52pm. By this time the Moon has sunk a further five degrees closer to the horizon.

       

         It is also possible observe a grazing occultation of zeta at the northern limits of the occultation. This type of occultation only occurs within a mile or two of the northern limits and if you are fortunate enough to be under this narrow path then the star will appear to blink on and off at random as it disappears behind lunar mountains and reappears in lunar valleys. 

       For the event on 25 April, the graze track cuts across Coll and northern Kilmarnock in Scotland, heads north of Carlisle and York , through Hull at 9.37pm BST and out to the North Sea. Observers to the south of this line will see a total occultation of varying durations depending how far away from the line they are, while for those to the north the Moon will slip south of zeta Tauri.