Our Place In Space

One of the central teachings for every Astronomy and Earth Science student is a clear picture of where the Earth is in space, what planets are our neighbors, where is the Sun and Moon relative to us, and the relative distance to stars in our galaxy and other galaxies. This page hopes to show you our home planet in the perspective of 1) The Moon, 2) The Sun and other planets, 3) The Stars in the Milky Way, and 4) The Universe. Since much of this information is already contained in greater detail earlier in this course, or later, I will try to keep my comments concise.

Where is the Moon relative to our Planet?

The Moon is our only natural satellite. It is roughly the diameter of the contiguous 48 states of America (diameter of 3476 km). The Moon orbits the Earth at an average distance of 384,400 km (238,300 miles). Just as all planets orbit the Sun in an elliptical orbit (Kepler's First Law), so too does the Moon orbit the Earth in an elliptical orbit. When the Moon is at its closest approach to the Earth, it is at a point called Perigee, which is 356,410 km (221,460 miles). When the Moon is at its farthest distance from the Earth, it is at a point called Apogee, which is 406,700 km (252,700 miles). The tidal influence of the Earth-Moon-Sun system causes variations in the apogee and perigee, and thus the Moon's distance varies. To learn more of the changes, see the Apogee/Perigee page. The effects of this ellitpical orbit are seen in the Moon:Tides unit. While the distance seems pretty big, it is difficult to notice the effects on the Moon as it appears in the sky. A photographer with a moderately-sized telescope can take pictures of the Full Moon at apogee and perigee and compare the effect on the apparent size of the Moon, and the effect is more discernable. Click on the image below to learn more about this picture from APOD.

To learn more about the Moon, check out the pages devoted to its study in this course.

Where are the Sun and other Planets relative to our Planet?

The Sun is the central feature of our Solar System. While early philosophers and naturalists believed the Earth was the center of the Universe, Copernicus, Kepler, Galileo, and Newton were effecitve replacing our beloved home planet (with its philosophy of self-centeredness) with the Sun as the center of attention and the object that all of the planets orbit. The Sun is nothing more than an average, middle-age star. Yet, this hot ball of gas provides the light and heat upon which all of our lives depend and around which the planets orbit. The Sun is 1.3 million times bigger than the Earth, but because it is 149,600,000 km (93,000,000 miles) away, it seems sort of small in the sky. The Sun's family includes 9 planets (astronomers are debating what to term a large object found in 2005 that appears to merit the title of planet). Earth occupies the 3rd position out from the Sun, with Mercury and Venus orbiting inside our ellipse (inferior only in definition of their place relative to ours). Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto (for 228 out of every 248 years) orbit outside our ellipse (superior only indefinition of their place relative to ours). To gain a perspective on how far away these planets are from us, how far we are away from the Sun, and just how tiny our planet is in the vastness of space, you are asked (not encouraged, but asked ... expected, commanded, urged, begged and groveled) to make a Scale Model of the Solar System and "see" where everything is relative to us. To get a perspective of the Solar System from any place within the Sun's family, go to the JPL site. The image below shows the Sun and the planets ... along with a comet and a tiny representation of the Moon. None of the objects are in relative size or distance. For that perspective, you need to do the Scale Model.


Where are the Stars in the Milky Way relative to our Planet?

When you look up into the night sky and see the stars (something that is increasingly hard to do in the city), it seems as if you could almost touch them. When you watch television and movies about star-hopping starships that move a warp speed, hyperspeed, or even ludicrous speed, this feeling of close proximity to the stars is only reinforced. In reality, the stars are really far away. If you consider that the light from the Sun gets to the Earth in a little more than 8 minutes and arrives at Pluto in roughly 4 hours, you seem to think that the stars cannot be much farther than that ... perhaps a few more hours away. You will discover that the nearest stars are no light hours, days, or weeks, or even light months away from us. The stars are at distances measured in light years! If you complete your Scale Model of the Solar System, the final request is to locate Proxima Centauri on your scale model. This star is the closest star among the 200,000,000,000 stars in the Milky Way. If your hike of the scale model required you to walk for 5 minutes at a brisk pace to get at the scale distance to Pluto, you could continue walking at that pace for 33 days (non-stop) and arrive at the next star! Space is Huge, and our planet is a tiny dot in comparison. The Sun, Earth, and other planets are orbiting in a spiral arm of the Milky Galaxy at a distance of 30,000 light years from the galactic core. The Sun is just one of 200 billion other stars moving around this core, and many of the stars are as far from each other as we are from Proxima Centauri. A nice HST image of a field of some local neighbor stars :)

Where is the Earth relative to the Universe?

The Milky Way is one of 20 galaxies that together comprise the "Local Group." This group of galaxies is made up of 2 giant spiral galaxies (The Milky Way and Andromeda) and small irregular galaxies with names or numerical designations. The Local Group is part of a larger group of galaxies called the Virgo Cluster. This group holds about a dozen local groups. The Virgo Cluster is one of several galaxy clusters that together make up the Virgo Supercluster. And, the Virgo Supercluster is one of a string of other superclusters that makes up a Filament. The Universe is a network of long Filaments that outline huge voids of empty space, as seen in the image below.

Source: Richard Powell

If you like this picture, you have got to check out more of the Atlas of the Universe. It is an incredible perspecitve of our place in space, and will give you the picture that accompanies my words.

After you have completed your study of Earth's place in space, move forward to Earth Facts, or take a break to mentally prepare for the fascinating material you will study as you learn about the Earth by going back to the Syllabus or the Home Page.

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