Southern Cross Viewed from Tenerife
September 05, 2018
Photographer: Jurgen Rendtel
Summary Author: Jurgen Rendtel
Can you spot the Southern Cross, one of the most conspicuous constellations, on the above image? During my visits to the Observatorio del Teide at Tenerife, Canary Islands, Spain, I'm occasionally asked whether the Southern Cross constellation is visible completely. Since the observatory's latitude is 28 degrees 18 minutes north, the first order magnitude star Alpha Crucis (Acrux) in the Southern Cross, with a declination of -63 degrees 6 minutes remains 1 degree 24 minutes below the southern horizon. So the first answer is "No".
But there are two effects which modify this general geometrical situation. First, there's refraction that acts to lift the position of an object at the horizon by 34-39 minutes, when observing from sea level - depending on the atmospheric layering (temperature and density), which can't be known until the time of observation. Note that at higher altitudes the refracting angle is lower. As a rule, at 5,000 m altitude, the angle is half the value found at sea level. Because the observatory altitude is 2,380 m above the level of the sea, we may assume that the refraction is about 25 minutes. Thus not sufficient to make the star Acrux visible.
Second, altitude increases the visibility range enormously. From the top of the observatory we find that the tangential beam reaches sea level in about 190 km (our sight line extends 190 km). So we're able to look below the mathematical horizon (the horizon if we were viewing from sea level) by about 45 minutes -- provided that the line of sight is free from low clouds. Typically, some low clouds form around the Canary Islands, and therefore the amount we can see below the horizon isn't fully realized. But if it's clear, we gain 25 minutes from refraction plus 45 minutes from the altitude, shifting the position south by a full 1 degree 10 minutes. However, this still isn't enough to make Acrux visible.
So to see Acrux a more significant atmospheric refraction is required. I observed such a strong refraction on May 14, 2018, as shown above (click to see annotated version). This image is one of a series taken with a Peleng fisheye lens. In essence, the altitude and strong atmospheric refraction shifted the southern horizon at the meridian near Acrux so that this long exposure photo included the grouping of stars known as the Southern Cross.
Photo Details: Canon EOS 60Da camera; Peleng fish eye lens; f/8 mm; 59-second exposure.