A Quick Tour The distance between Baltimore and Washington is roughly 60 km. Let’s make a new unit of measurement called a BW where, 1 BW = distance between Washington and Baltimore Earth - Diameter about 13,000 km or 213 BW Earth’s Moon – about 380,000 km away or 6,400 BW The Inner Solar System (Sun, Mercury, Venus, Earth, Mars) – about 3 Astronomical Units (AU) or 450 million km across or 7.5 million BW The Solar System out to Pluto’s orbit 80 AU or about 12 billion km across about 200 million BW Nearest 25 stars enclose a region about 26 light years or 2.5 x 1014 km across 4 x 1012 BW = 4 trillion BW. Our “neighborhood” within a spiral arm of the Milky Way Galaxy – a few kpc or about 9 x 1016 km about 1.5 x 1015 BW. The Milky Way is about 100,000 light years or 30 kilo parsecs across. This is about 1.5 x 1016 BW. The nearest galaxy similar to our own Milky Way Galaxy is the Andromeda Galaxy. It is 670 kpc or about 2 million light years away. This is 3.4 x 1017 BW. Our galaxy and Andromeda belong to a cluster of about 30 galaxies called the Local Group about 3 million light years across. The Local Group is a member of the Local Supercluster about 30 mega parsecs or 100 million light years across. The most distant object observed is a supernova explosion roughly 10 billion light years away or about 1 x 1021 BW.