A Spectacular Solar Eclipse

A Spectacular Solar Eclipse

A Solar Eclipse is perhaps the most impressive of all astronomical events, and we owe it all to fortuitous coincidence. Once about every six months the Moon passes directly between Earth and the Sun. The coincidence is that although the Sun is about four hundred times larger than the Moon, it is also about four hundred times farther away, so the two objects appear nearly the same size in our sky.  As a result, during a total eclipse the Moon just covers the solar disk.

When that happens, the Moon casts a shadow of Earth’s surface. Unfortunately, the shadow covers only a narrow strip of Earth. For those in the path of the shadow, the early partial phases of the eclipse are mere prelude. In the seconds just before the Sun is completely engulfed, viewers see the last vestiges of the Sun’s light peeking between mountain peaks on the Moon’s limb, a phenomenon known as Baily’s Beads. During totality stars pop into view as the sky is plunged into near-darkness.

Galileo Flies above Jupiter’s Cloudtops

When the eclipse arrives, for those who have always wanted to visit a country where it can be seen,, but never had a good excuse, this moment is a great time to go. For instance, on July 11, 1991, Hawaii was one of two prime observing spots for what was the best solar eclipse in the 1990s. The July 11, eclipse began in the Pacific Ocean west of Hawaii and moved ashore on the Big Island about 7.30 a.m. local time. At 11.45 a.m. the shadow crossed the southern tip of Baja California. Totality lasted almost seven minutes there – nearly the maximum length of time possible for a total eclipse. The Moon’s shadow raced on through central and southern Mexico, down the Pacific coast of Central America, and into Colombia. Finally it crossed into the Brazilian rain forest and then left Earth just short of the Atlantic Ocean.

The Solar Orbit

The Solar Orbit

In the case of the solar orbit it is the Sun’s gravity that is pulling on the satellite and making it go around and around the Sun. Of course, all the orbits are also solar orbits, because the spacecraft that is orbiting the Earth or even sitting on the Earth before it is launched, is already in a solar orbit – along with the Earth and everything on it, or going around it.

So when we want to place a spacecraft into a solar orbit, what we really want to do is place it in a solar orbit that is different from the one it is already in – namely, the Earth’s orbit. Usually we want to do this to make the spacecraft travel through the solar system to another planet, such as Mars or Venus, but sometimes (as in the case of Pioneer spacecraft) we just want to find out what is out there in interplanetary space.

The principles involved are just the same that are involved when satellites change from one orbit to another. We are already in one solar orbit (the Earth’s) and we simply want to change to another. If we want to change to a lower orbit, which means one nearer the Sun, we have to slow the spacecraft down from the Earth’s orbital speed. We do this by firing a rocket engine in the opposite direction to the Earth’s motion around the Sun. This slows down the spacecraft from the Earth’s speed to a lower speed, and it drops to a lower orbit, inside the Earth’s. It is still going in the same direction, remember, but closer to the Sun. If we had a powerful enough rocket to cancel out all of the Earth’s speed, our spacecraft would be stopped still and would simply fall right into the Sun.

Neptune and Its Great Dark Spot

If we want to go into an orbit that is outside the Earth’s orbit – which is what we have to do to go to Mars, for example, we fire the rocket engine in the same direction that the Earth is going. This gives it a push (the rocket fires for only a few minutes) ahead of the Earth, and it costs out of the higher (father from the Sun) orbit.

In either case, whether we want to go to a lower solar orbit or a higher one, the spacecraft has to escape from the Earth’s gravitational force. And to do this, it must reach escape speed, which is a little over 25,000 per hour. Of course, the farther out (higher) or in (lower) we want the spacecraft to go, the more we have to speed it up beyond this escape speed.

And remember, just as with Earth orbits, even though you slow a spacecraft down from Earth’s orbital speed around the Sun in order to make the spacecraft fall into a smaller solar orbit, by the time it falls to that orbit it is going faster than when it left the Earth. And even though you speed a spacecraft ahead of the Earth in order to push it into a larger solar orbit, by the time it gets there it is going more slowly than before. That is why satellites that are launched “behind” the Earth into inward path eventually overtake and pass the Earth, and satellites that are launched ahead of the Earth into outward paths eventually fall behind.

The Moon Covers the Pleiades

The Moon Covers the Pleiades

The slender crescent Moon plays a game of astronomical tag with a cluster of bright young stars, the Pleiades, on the evening of March, 20. In fact, the Moon appears to “catch” the Pleiades, hiding the stars from our view, before it sets.

Of course, the Moon only appears close to the Pleiades. The cluster of several hundred stars is really about 400 light-years from Earth, some 10 billion times more distant than our Moon. Still, the view should be impressive, particularly through a good pair of binoculars.

The Moon Passes the Pleiades Star Cluster. Image credit: The University of Manchester / Derekscope, with the Moon from a 2009 conjunction.

The Pleiades is one of the loveliest naked-eye objects in the night sky. Six tightly grouped stars in the shape of a small dipper are normally visible to the naked eye and dozens more can be seen with binoculars. The brightest star in the Pleiades is Alcyone, a blue-white giant several hundred times more luminous than our Sun.

Look for the Moon and the Pleiades fairly high in the west as twilight deepens on March 20. Depending on what part of North America you live in, the Moon already may appear in front of the cluster. As the evening progresses the Moon keeps moving relative to the cluster and many of the cluster’s stars will be occulted, or hidden from view, by the Moon. Because the Moon has no atmosphere and the stars appear as pinpoints of light, the individual Pleiades disappear instantly when the dark limb of the Moon passes in front of them.

The Pleiades cluster, named for the mythological Seven Sisters of the Pleiades (the daughters of Atlas), contains several hundred stars. The Pleiades became known as the Sailors’ Stars during classical times when the cluster ascended into the eastern sky at the beginning of the Mediterranean Sea’s calm-weather season.

Inclination of the Orbit Plane

Inclination of the Orbit Plane

The satellite’s path is always in a plane; that means you could draw it on a piece of paper. And whatever kind of orbit we’re talking about, its plane always goes through the center of the Earth.

An orbit plane can be inclined at any angle between equatorial and polar, and we choose the inclination to suit whatever purposes we have for the satellite. Sometimes we choose the inclination that is the easier – that is so we can put the most payload into orbit with a given rocket. Many US satellites and manned spacecraft are put in orbit inclined at or near 33 degrees to the equator, because that is the easiest inclination for launches from Cape Canaveral (which is at 33 degrees latitude). Many Russian satellites are in orbits inclined in the neighbourhood of 57 degrees to the equator because that is the easiest for them; their principal launch site is at 57 degrees latitude. In both cases that is the easiest inclination because it takes advantage of the rotation of the Earth; you launch the satellite in the same direction that the Earth is already rotating (east), and that gives you some extra push.

Loops of Emission Nebulosity in NGC 3576. Photo in public domain

Putting the same satellite into a polar orbit takes a more powerful rocket, because now you get no benefit at all from the spin of the Earth. And unless you can launch from the Equator, putting it into an equatorial orbit takes some extra thrust because you have to change the plane of the orbit by firing a rocket engine in the right direction just as the satellite is crossing the equator.

Polar orbits are useful for weather satellites, because while the satellite is going around and around over the north and south poles, the Earth is turning below, inside the satellite’s orbit, and each north-south (or south-north) pass of the satellite scans a new part of the planet’s surface until it has all been scanned. If you make the orbit almost polar but not quite, you can make the orbit plane itself rotate at the same rate the Earth is going around the Sun (about 1 degree per day). This near-polar plane is also useful for weather satellites because if you start at the right time you can keep the Sun always behind the satellite. This will allow the satellite’s cameras to take pictures in daylight during the whole year.

For communication satellites like TRW’s Intelsat III, it is best to use a so-called geostationary or synchronous orbit. We already saw that there is a particular altitude for each speed, and therefore for each period (time it takes for one revolution around the Earth), from the hour-and-a-half period of Mercury spacecraft to the 28-day period of the Moon. Somewhere in between there must be an altitude that will give you a period of exactly 24 hours, and this turns out to be a little over 22,000 miles. If you put a satellite in circular orbit in the plane of the equator at that altitude, it will be going around the Earth at the same rate the Earth is going around its own axis. That means that the satellite will remain over the same place on the Earth at all times, and to someone on Earth it would appear to be standing still.

Unveiling the Universe

Unveiling the Universe

Galileo Galilei, the great Italian astronomer and mathematician, was born on February 15, 1564. Although his ideas were condemned as heresy by the Roman Catholic Church, Galileo revolutionized man’s concept of Earth’s place in the heavens. He did so by providing strong observational evidence supporting the views of Copernicus, who said that the Sun, not Earth, is the center of the solar system.

Galileo was the first person to use an extraordinary new invention – the telescope – to examine the heavens. With his telescope Galileo discovered four bright points of light circling Jupiter. The objects were Jupiter’s four largest moons: Io, Europa, Ganymede, and Callisto.

Galileo Galilei. By Giusto Sustermans, 1636

This finding was a key piece of evidence in support of the Copernican view of the heavens. Others had argued that if Earth really orbited the Sun our Moon could not keep up. By demonstrating that other moons could orbit another planet that everyone agreed was in motion itself, Galileo disproved this notion.

Galileo also turned his instruments on Venus, our nearest planetary neighbor. Again his telescope revealed crucial evidence in defense of the Copernican view of the solar system: Venus, like Earth’s Moon, exhibits phases. If the Sun and planets orbited Earth, Venus would always appear as a crescent. But Galileo found that Venus displays both crescent and gibbous phases, meaning that it must be in orbit around the Sun.

Galileo’s work is honored and respected today, but in the seventeenth century it created major problems for the astronomer. After an appearance before the Roman Inquisition he was forced to deny his own findings and was placed under house arrest for the last decade of his life.

Flowers are part of our universe. Illustration by Elena.