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| Favorite Star Patterns on Apr 15, 2014

Southern Cross: Signpost of southern skies

The constellation Crux – otherwise called the Southern Cross – can be seen from Earth’s Southern Hemisphere and from tropical and subtropical regions of the Northern Hemisphere.

How to find it

At the tropic of Capricorn (23.5 degrees south latitude) and all latitudes farther south, you can see the constellation Crux – otherwise known as the Southern Cross – at any hour of the night all year around. In that part of the Southern Hemisphere, the Southern Cross is circumpolar – always above the horizon. However, for much of the northern hemisphere – including most of the United States – the Southern Cross never rises above the horizon, so it can never been seen from our middle and far northern skies.

Crux, aka the Southern Cross. Photo credit: ESO/Yuri Belesky

To see all of Crux from the US, you have to be as far south as Hawaii, or the southern parts of Florida or Texas (about 26 degrees north latitude or farther south). Even from these spots in the US, you have a rather limited viewing window for catching the Southern Cross. It has to be the right season of the year. It has to be the right time of night. And you have to look in the right direction: SOUTH!

For the Northern Hemisphere’s tropical and subtropical regions, the month of May is a good time for finding Crux in the evening sky. You can see the Southern Cross at other times of the year, but not at such a convenient time. In middle March, for instance, you have to wait till about 1:00 a.m. to catch the Southern Cross at its highest point in the sky. In December and January you have to catch Crux before dawn.

No matter the hour or date, the Southern Cross climbs to its highest point in the sky when it’s due south. The Cross is fairly easy to visualize, because it stands upright over the horizon.

Enter the Big Dipper

Although the Big Dipper is a fixture of the Northern Hemisphere skies, this star formation has a close kinship with the Southern Cross. Both the Big Dipper and the Southern Cross reach their highest point in the sky in unison. Remember spring up and fall down. The Big Dipper soars highest in the sky on late spring evenings. When the Big Dipper is seen above Polaris, the North Star, the Southern Cross is seen standing over the southern horizon in the southern Florida and Texas.

For the Southern Hemisphere, it works the same way – but in reverse. The Big Dipper can actually be seen in the Southern Hemisphere from about 26 degrees south latitude and all latitudes farther north. But to spot it, the Big Dipper has to be viewed at the right season of the year and the right hour of the night. When the Southern Cross sails highest up in the Southern Hemisphere sky, the upside-down Big Dipper is seen just above the northern horizon at latitudes nearby the tropic of Capricorn (23.5 degrees south latitude).

Here is the Southern Cross as seen from Manila – latitude 14 degrees N. of the equator – on an April evening in 2012. The photo is from EarthSky Facebook friend Jv Noriega. Thank you, Jv! Make larger.

Navigation and Science

When European sailors journeyed south of the equator, they found that the North Star had disappeared below the horizon. As they sailed even farther south, the Big Dipper dropped out of sight as well. Fortunately, the Southern Cross acts as a navigational aid. A line drawn from the star Gacrux through the star Acrux points in the general direction of the south celestial pole – the point in the sky directly above the Earth’s south pole. Unlike the Northern Hemisphere, the Southern Hemisphere has no appreciably bright pole star to highlight the celestial pole.

Astronomers have plotted the pathway of the galactic equator across the starry heavens, finding that the equator of the Milky Way tilts about 60 degrees to the celestial equator. The celestial equator is a projection of the Earth’s equator onto the stellar sphere. The southern terminus of the galactic equator passes near Crux’ southernmost bright star, Acrux.

Rising and setting time for the Southern Cross star Mimosa into your sky

Mimosa is second-brightest star in Southern Cross

Acrux is brightest star in Southern Cross