Two planets stand out in March’s evening sky, but let’s start our tour with a pair of lesser lights that hug the western horizon. Although Venus lies a respectable 30° from the Sun at the beginning of the month, it appears low in the sky. On the 1st, you can find the planet 5° high a half hour after sundown. Gleaming at magnitude –4.8, it’s conspicuous if you have an unobstructed horizon.
The inner world’s low altitude arises because the ecliptic — the Sun’s apparent path across the sky that the planets follow closely — makes a shallow angle to the western horizon after sunset at this time of year. Venus also lies a few degrees north of the ecliptic, which compounds the problem. Its altitude improves slightly the closer you live to the equator. Try to grab a quick look through a telescope in early March. You’ll see a disk measuring some 50″ across with a wafer-thin crescent phase.
Venus quickly disappears in the twilight glare as it heads toward inferior conjunction March 23. It will return to view before dawn in April.
Mercury also lies in the evening sky, reaching a greatest elongation of 18° March 8. But it glows much more dimly than Venus and hangs even lower, making it nearly impossible to see from mid-southern latitudes. Like Venus, Mercury puts on a dramatically better show in April’s morning sky.
Fortunately, two outer worlds make up for the poor performances of the inner planets. You can find Jupiter about one-third of the way from the northwestern horizon to the zenith as twilight comes to a close. Shining brilliantly at magnitude –2.2 in mid-March, you won’t have any trouble identifying it among the background stars of Taurus the Bull.
Jupiter maintains a robust angular size — 38″ at midmonth — and looks spectacular when viewed through any size telescope. The giant planet’s banded atmosphere explodes with detail during moments of good seeing. And you’ll easily spot the four bright Galilean moons as they change positions from night to night.
The ruddy glow of Mars, embedded among the background stars of Gemini, appears nearly due north in the early evening sky. Although the Red Planet dims from magnitude –0.3 to 0.4 during March, it still shines brighter than any of the Twins’ stars.
The fading of Mars coincides with its increasing distance from Earth. This means the outer world also grows smaller. The planet’s disk shrinks from 11″ to 8″ across during the month. Patient observers still should see some dark surface markings when the seeing steadies.
The final naked-eye planet, Saturn, reaches conjunction with the Sun on March 12 and remains out of sight all month.
A total lunar eclipse occurs March 14. Observers in South America and the eastern Pacific Ocean will have the best views. The eclipse’s umbral phase lasts from 5h09m to 8h48m UT while totality runs between 6h26m and 7h32m UT. The final stages also will be visible from New Zealand and eastern Australia.
A partial solar eclipse takes place March 29, but it is strictly a Northern Hemisphere event.
The starry sky
The famous Greek astronomer Hipparchus introduced the magnitude scale more than 2,000 years ago. He assigned the brightest stars 1st-magnitude status and the faintest 6th-magnitude status. In the mid-19th century, astronomers made this scale more rigorous by defining a difference of five magnitudes as corresponding to a factor of 100 in brightness.
In keeping with Hipparchus, observers generally consider stars of magnitude 6 the faintest visible to the unaided eye while the brightest (other than the Sun) are often called 1st-magnitude stars. Because of the strict mathematical definition now used, the brightest objects actually have negative magnitudes. Sirius, the most brilliant star in the night sky, has a magnitude of –1.46.
Even though a handful of stars possess negative magnitudes, many amateur observers loosely but conveniently consider any star at least as bright as magnitude 1.50 as a 1st-magnitude star. Because of rounding, a star of magnitude 1.51 would be relegated to 2nd-magnitude status.
Intriguingly, both the brightest and faintest of these so-called 1st-magnitude stars reside in Canis Major the Big Dog. This constellation passes nearly overhead early on March evenings, so now is a great time to examine the pair. I’ve already mentioned brilliant Sirius; Adhara (Epsilon [ε] Canis Majoris) marks the other end of the scale. It comes in right at magnitude 1.50, making it the 22nd and final 1st-magnitude star.
Within the past decade, the International Astronomical Union has instituted a significant change in the naming of stars. The organization now assigns names just to individual stars. This means that only one of the members in a binary or multiple system carries the name that may have been traditional for a long time. Epsilon CMa is such an example — a binary system hosting stars of magnitude 1.5 and 7.5. Only 7″ separate the two and they differ in brightness by a factor of 250, making them tough to split in amateur instruments.
A noteworthy point about Adhara and its companion: They currently are receding from Earth at 27 km/s, so they are gradually fading. There eventually will come a time when Epsilon falls off the 1st-magnitude list.
Star Dome
This map portrays the sky as seen near 30° south latitude. Located inside the border are the cardinal directions and their intermediate points. To find stars, hold the map overhead and orient it so one of the labels matches the direction you’re facing. The stars above the map’s horizon now match what’s in the sky.
The all-sky map shows how the sky looks at:
10 P.M. March 1
9 P.M. March 15
8 P.M. March 31
Planets are shown at midmonth
