Category: Sky Events

Sky events

Mars Opposition 2012

About a month from today, on March 3rd, the planet Mars will pass through opposition.

Mars orbits the Sun every 1.88 years, with Earth only taking one year for each orbit. Like two runners on a track the two planets race each other around the Sun. But we have the inside lane, lapping the red planet every two years. These events are called opposition, when Mars is closest to us and best positioned for viewing by earthbound telescopes.

Closest approach of the two planets is not necessarily on the same day as opposition, but can vary up to two weeks. This year closest approach will occur March 5th with the two planets approaching to 99,331,411 km (61,721,554 miles) at 07:01HST.1 At this distance the red planet will show a disk 13.89″ arc-seconds across in the eyepiece.2

Mars during the 2005 oppositionMars during the 2005 opposition

Do not worry about viewing on the 3rd or 5th, any time in the month leading up to and after opposition the viewing will be very good. With even a modest telescope it should be possible to see the bright polar caps and light and dark markings on the planet.

All month Mars will be visible throughout the night, high in the sky at midnight. This is the time to enjoy observing our closest neighboring planet while it is nearby and high in the night sky.

1) 2012 Mars Opposition at SEDS
2) The 2012 Aphelic Apparition of Mars

The Moon and Pleiades

Tonight a bright gibbous Moon will be just under 5° from the Pleiades star cluster. The Moon will be 62% illuminated, bright, but the cluster is bright enough to be seen even against a bright Moon. As the Pleiades move to the west over coming months there will be a few more lunar conjunctions, with increasingly smaller crescents.

The Moon and Venus

This evening a nice crescent Moon will join Venus in the dusky sky. The pair will be reasonably close, about 8° apart. Venus is currently shining very brightly at about -4.1 magnitude, contrasting nicely with a 9% illuminated Moon. The two will still be close tomorrow, about 9° apart with the Moon 15% illuminated.

Meteor Watching

Watching meteors is one of the simplest forms of astronomical observing. Just about anyone can enjoy meteor watching, from just about anywhere in the world. Enjoying the show takes only a couple things… A dark sky and a comfortable place from which to watch.

Leonids in Orion
A pair of Leonid meteors streak through Orion
Meteors are simply small bits of debris hitting the Earth’s atmosphere at very high speed, typically tens of thousands miles per hour. Our solar system is rich with this debris. Most of these bits are quite small, about the size of mote of dust or a grain of sand. Something the size of a pea would create a spectacular fireball that lights up the whole sky. While they often seem close, they are actually quite high, 60 miles (100km) above the ground when they flare into short lived fireworks.

The mechanism for the show is simple. When something hits the very thin air high in our atmosphere at very high speed it compresses the air in front of it. This compression also heats the air, causing it to glow white hot. Heated enough, the air becomes a plasma, the molecules shredded and electrons freed from the atoms. It is not the meteor itself that you see, but the glowing plasma around it.

There are a number of questions many people ask about meteor observing. You can find many of the answers below. Watching a meteor shower takes no special equipment, expert knowledge or extravagant preparation. This is an activity nearly anyone can enjoy, one of the spectacles of nature available to all.

Continue reading “Meteor Watching”

Lunar Apogee and Perigee

Does the Moon seem a little larger and brighter than usual? It may not be an illusion, sometimes the Moon really does look a little larger or smaller in the sky.

Like all orbiting objects, the Moon does not orbit in a perfect circle, but rather in an ellipse. This means that as it orbits it is a little further away or a little closer. In the case of the Moon the difference is not much, but you can see it, if you know to look.

Lunar Apogee & Perigee
The apparent size of the moon as seen at perigee versus apogee
When the Moon is furthest from the Earth, a point called apogee, it will be about 405,000km (251,000miles) from the Earth. While at perigee, the Moon will be about 360,000km (223,000miles) from the Earth, as measured from the center of the Earth to the center of the Moon. The change in distance leads to a noticeable difference in the size of the Moon as seen by an observer here on Earth. When at perigee the moon will appear about 12% larger than when seen at apogee.

Lunar Month Animation
Click to view a simulation of a lunar month showing phase, libration (tilt), and the size change as the Moon goes from apogee to perigee. Image credit Tomruen via Wikimedia Commons
The difference is most noticeable at full Moon. If the full Moon occurs near apogee or perigee, an experienced skywatcher can spot the difference. The effect should not be confused with the well known Moon illusion, where the Moon can appear larger near the horizon.

The period of time between full Moons, the synodic month, is about 29.5 days. While lunar perigee occurs every 27.5 days, an anomalistic month. Since these periods are not equal, the cycle drifts in and out of phase. About once a year the cycles coincide and full Moon and apogee or perigee will occur near the same time.

At apogee, the Moon will be appear about 29 arc-minutes in size, a little less than half a degree. At perigee the Moon will be about 33 arc-minutes across, a bit more than half a degree. The numbers may not seem like much, but it is a visible difference. The simulated images shown here will give a better idea of what the numbers represent.

This change in size and distance leads to the moonlight being a bit brighter at perigee than at apogee, about a 30% difference. So if that moonlit night seems brighter than you remember it may actually be the case.