Using Organic keywords Report in Site Explorer

First, go to the Organic keywords report under Organic search in Site Explorer:

There, you can compare the ranking history difference between two time periods of your choice.

You can use the date pickers to change the date and comparison date:

You can also use many of the filter options to narrow down specific keywords you'd like to compare:

For each keyword, you can also click the Chart button to see the Position history of how pages of the Target URL have ranked for this keyword on Google SERPs over time.

Highlighted in Pink, you can also change the duration to view the chart, and the number of pages that can be compared to on the graph.

How far back you can view ranking history for a keyword depends on your subscription tier:

If you've already added keywords to a project in Rank Tracker, you can view its ranking history via the Overview report and Ranking history report. If you haven't, this would be a good opportunity to start adding keywords to Rank tracker now.

Rank tracker provides all the same features available in the Organic keywords report previously mentioned: Date pickers, Filters, Position history and SERP comparison:
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There's an additional benefit in the Position history graph, where you can toggle between Desktop and Mobile rankings:

How far back you can view ranking history depends on when you first added the keyword into Rank Tracker. This can be further back than the Historical data limits your current subscription's tier has, as those only applies to Site Explorer and Keywords Explorer.