Astronomy Ch01.6: Key Concept: Our Cosmic Origins

Part D On the cosmic calendar, which compresses the history of the universe into a single year, about when did life arise on Earth? - in September

From Part C, Earth formed in early September on the cosmic calendar. Life apparently arose on Earth within less than a billion years after that, which means it was still in September (because each month on the cosmic calendar represents about 1.2 billion years).

Part A - Ranking Task Consider the indicated events in the history of the universe that have helped make human life possible. Rank the events based on when they occurred, from longest ago to most recent. To rank items as equivalent, overlap them. Note: If two events occurred within seconds of each other, rank them as equivalent. - the Big Bang & the universe begins to expand - elements such as carbon and oxygen first exist - nuclear fusion begins in the Sun - earliest life on Earth - dinosaurs go extinct - earliest humans

Now that you know the order in which these events occurred, continue on to check whether you know approximately when they occurred.

Part B According to current scientific estimates, when did the Big Bang occur? - about 14 billion years ago

Scientists estimate the age of the universe by measuring the expansion rate, from which they can determine how long it has taken the universe to reach its current size. Current estimates put this age at about 14 billion years.

Part E On the cosmic calendar, which compresses the history of the universe into a single year, about when did early humans first walk on Earth? - just a few hours before midnight on December 31

The cosmic calendar compresses the 14-billion-year history of the universe into 1 year, which means that each day represents about 1/365 of 14 billion years, or about 40 million years. Early humans arose just a few million years ago, which therefore means just a few hours before the present moment (the stroke of midnight on December 31) on the cosmic calendar.

Part C On the cosmic calendar, which compresses the history of the universe into a single year, about when did Earth form? - in early September

The cosmic calendar compresses the 14-billion-year history of the universe into 1 year, which means that each month represents about 1.2 billion years. The solar system's age of 4 ½ billion years therefore puts its birth in early September.