Space Travel Diary

Author: Carl Sagan
Publisher:
Publish Date: 2004-03-01
Features: At first, there was an unimaginable big bang. Later, our galaxy was born. Now, we are racing through the universe at the speed of light, chasing thousands of other celestial bodies—stars, satellites, meteors, comets, and asteroids. The Travelers' Guide to the Galaxy will tell you about the almost magical events of the galaxy we inhabit. It's guaranteed to make you jump out of your seat, like stepping onto a giant solar mirror. In the book, you can ski on Mars, meet grieving travelers from Pluto, and learn incredible information about red giants, white dwarfs, and supernovas. Finally, you'll discover what happens if you get too close to a black hole.
Very Bright Stars
-1.6 is the magnitude of Sirius, the brightest star in the sky.
-4.4 is the greatest magnitude of Venus.
-12.5 is the magnitude of a full moon.
-26.7 is the magnitude of the Sun.
From Earth, Sirius appears as the brightest star in the sky because it is very close to us—only 8.5 light-years away. Other stars are much more luminous than Sirius (such as Rigel in Orion), but because they are extremely far away, they appear less bright.
How to Find Planets
Since planets move across the sky, marking them on a star chart is nearly impossible. However, there are exceptions. Every year, people publish various "almanacs" and charts, the best of which is The Sky at Night. This book includes a star chart that highlights the positions of planets for each month of the year, as well as information about the moon's path and whether beautiful lunar eclipses will occur. If you can locate the planets independently, it’s truly exciting! Once you are familiar with different constellations, you can carefully observe which constellation suddenly has an extra star. (Make sure it’s not a plane, a UFO, or anything else.) This star is not a star—it must be a planet! Because planets have their own orbits, sometimes you won’t see a planet for weeks, while other times you can spot several at once.
Insider’s Ink
Mercury and Venus are called inferior planets. Does this mean they are of low status, even allowing goldfish to give them orders? Not at all—the term simply refers to the fact that they are closer to the Sun than Earth. Because they are close to the Sun, they hang in the sky all day. However, the Sun’s light is so bright during the day that they are almost impossible to see. Of course, when they are in certain positions, we can sometimes spot them shortly before sunrise or after sunset. If Mercury and Venus align perfectly with the Sun, we won’t be able to see them at all. (This alignment is called conjunction.) In this diagram, Mercury is between the Sun and Earth. You might think we should be able to see it, but in reality, like trying to spot a grain of sand in the glare of a car’s headlights, you won’t be able to find Mercury. When Mercury and Venus are on the same side of the Sun or on opposite sides, we can see them just before sunrise or shortly after sunset. Even in the best observing season, we can only spot Mercury within two hours before sunrise or after sunset. Unfortunately, it’s very hard to spot because it’s tiny and often near the horizon. (Observing objects low in the sky is difficult because the air near the Earth’s surface is turbulent, often making small objects appear blurry.) Venus is farther from the Sun than Mercury, so we can see it within three hours before sunrise or after sunset. Most months, whether in the morning