Sphere of Stars
Southern Cross constellation image © NASA.
JB: This is Earth and Sky for Wednesday, May 19. Over 2,000 years ago, Aristotle wrote of stars visible from Egypt that couldn’t be seen from his native Greece.
DB: He knew stars could be seen above Egypt’s southern horizon that weren’t visible further north. Aristotle realized that Earth is a globe. Being on the northern or southern half of Earth is like being a horse with blinders on. We can only view half of the sphere of stars that surround our planet. We can’t fully view the southern stars from the Northern Hemisphere, or the northern stars from the south.
JB: European explorers sailing south of the equator discovered this fact as well. They watched the Big Dipper – the signature star formation of our northern skies – disappear to the north. Meanwhile, the most famous Southern Hemisphere constellation – Crux also known as the Southern Cross – came into view in the south.
DB: But it is possible for the Big Dipper and the Southern Cross to shine in the same sky. On May evenings, the Big Dipper and the Cross reach their highest points in the night sky in concert. At this magical evening hour, the Southern Cross can be seen at latitudes as far north as southern Florida or Texas.
JB: At the same time, the Big Dipper can be seen all the way south to mid–Australia. For star charts showing the Big Dipper and Southern Cross, come to earthsky.org. Special thanks today to Research Corporation, a foundation for the advancement of science. We’re Block and Byrd for Earth and Sky.
