Cancer, the Crab
The ecliptic explained, plus the planets for April.
Almost overhead on April evenings is the constellation Cancer, the Crab. Made up of relatively faint fourth-magnitude stars, this constellation is not easy to find in an urban sky. In mythology, Cancer is most often associated with the giant crab that attacked Hercules as a distraction while he was battling the Lernaean Hydra. The story says that Hercules kicked the crab so hard that it flew up into the sky and became a constellation. Even after this distraction, Hercules was still able to slay the Hydra, which also ended up in the sky.
Despite not being very prominent in the sky, Cancer is one of the 12 signs of the Zodiac from astrology. These constellations (astronomer-speak) or "houses" (astrologer-speak) all have an invisible line in the sky called the ecliptic passing through them. Astrologers use the location of the visible planets in the sky to divine a person's future. Since the visible planets all stay near the ecliptic, they almost always can be found in one of the 12 Zodiacal constellations.
The ecliptic circles the sky like just like the celestial equator does, but it is tilted relative to the celestial equator by 23 degrees and 26 minutes. This puts the ecliptic far above the equator in the Gemini-Taurus border area and far below the equator in Sagittarius. Since Cancer is next to Gemini, the ecliptic is still well north of the celestial equator as it passes through the Crab.
One object that always stays right on the ecliptic is the Sun. So when the Sun is in the Gemini-Taurus area, the Sun is far north of the equator, putting it high in our sky. This gives us the long, hot days of summer in June and July. When the Sun is in Sagittarius, it is well below the celestial equator and we have the short, cold days of winter in December and January.
You may wonder why, of all the objects in our Solar System, only the Sun stays exactly on the ecliptic. If you think of the Earth riding around on the outside edge of the wheel of a toy gyroscope, the Sun is at the center of the gyroscope where the axis of rotation meets the spokes of the wheel. When they are spinning, gyroscopes will stand upright on their own, always keeping the edge of the wheel in the same plane. Projecting this into the sky, as we on Earth ride around with the edge of the wheel (orbit), we can look down toward the Sun and we will be looking down a spoke of the wheel along the plane of the wheel (ecliptic). The Sun must therefore be on the ecliptic, since it is the projection of the plane of the Earth's orbit and the "gyroscope wheel." If we could stand on the Sun, we would always see the Earth on the ecliptic as well, since they must both be in the same plane.
To have either a solar or lunar eclipse, the Earth, Moon and Sun must line up. Since the Earth and the Sun are always on the ecliptic, it is only the Moon that needs to be on the ecliptic and in front of the Earth or behind it to make an eclipse. The name "ecliptic" comes from the word "eclipse," because the Moon must be very near the ecliptic for an eclipse to occur.
The ecliptic is tilted up from the equator, because the Earth's axis is tilted from the plane of Earth's orbit. The ecliptic is a roughly in the same plane as the original dust and gas disc that eventually formed our Solar System. Because the planets all formed out of this same disc, they all orbit in roughly the same plane, so they always stay close to the ecliptic.
By now you are probably asking yourself: If the Sun, Earth and planets are all in the plane of that ancient gas disc, why is the Earth tilted over 23 degrees from that plane? The answer is the Moon. Some 4.5 billion years ago, a Mars-sized object struck a glancing blow on the primitive Earth. This knocked material off the Earth that eventually formed the Moon, and, in the process, tilted the Earth's axis to its current angle.
The Planets for April
Jupiter makes its swan dive into the evening twilight this month. At the beginning of the month, the King of the Gods is about 24 degrees up in the western sky as it gets dark. By the end of the month, it will be lost in the bright evening sky after sunset. Jupiter is moving slowly eastward in Aries this month. At midmonth, Jupiter's disc is just 33.2 seconds-of-arc across and it has faded to magnitude -2.0. Jupiter sets around 9:15 p.m.
Watch the Skies
The Goddess of Love still graces our evening sky, shining at magnitude -4.5. During the month, Venus will race eastward almost all the way across the constellation Taurus. At midmonth, Venus' disc is 38% illuminated and becoming more of a crescent. Venus' disc is 30.0 seconds-of-arc across and the planet sets around 11:15 p.m. Venus will be at its brightest for this apparition on April 30.
Mars is just past opposition this month. It starts the month moving westward in Leo, but on April 15, it stops and turns back eastward, staying in Leo all month. The God of War's disc is 11.2 seconds-of-arc across and it is magnitude -0.4 at midmonth. Mars is 50 degrees up in the east as it gets dark and it sets around 4:30 a.m.
The Ringed Planet has its turn at opposition this month, the same day that Mars is stationary, April 15. Saturn will be visible all night. Saturn's disc is 19.0 seconds-of-arc across, with the Rings tipped down 13.7 degrees showing their northern face and spanning 43.1 seconds-of-arc. Saturn is moving westward in Virgo, not far from the first-magnitude star Spica.
The Messenger of the Gods is the only planet in April's eastern morning sky. It reaches its farthest distance from the Sun on April 18, when it will be 27 degrees west of the Sun. At that time, Mercury's disc will be 46% illuminated, becoming fuller, and 7.9 seconds-of-arc across. It will be magnitude +0.4 as it rises at around 5:30 a.m. Mercury will be visible only for the last three weeks of the month. Mercury starts the month moving westward in Pisces, but it soon turns around, heading eastward, clips the corner of Cetus and then back into Pisces where it ends the month.
The April Lyrid meteor shower occurs at the dark of the Moon this year, making 2012 a good year to view this shower. The shower's radiant is in Lyra and the best observing time will be in the late morning hours while it is still dark. You will probably see about only 15 meteors an hour, but it can still be fun. This shower is the oldest recorded meteor shower, having been observed since 687 BC, according to Chinese records. The meteors are particles from periodic comet Thatcher C/1861. So stretch out under the stars with warm clothes and blankets for the celestial show and "keep watching the sky"!
An amateur astronomer for more than 40 years, Bert Stevens is
co-director of Desert Moon Observatory in Las Cruces.