Calculate Inter-twilight period

[chekos]

From the 2020 RIPA annual report:

Veil of Darkness methodology: The VOD technique examines stops that occur during a standardized inter-twilight period, or the time of day that is dark during Standard Time but light during Daylight Savings Time. By limiting the analysis to only those stops that occurred during this period, frequency comparisons are less susceptible to factors that vary by time of day (e.g... commuting patterns). To identify the inter-twilight period for the 2018 data, we sourced civil twilight times for each stop date and location using the United States Naval Observatory database.32 We bounded the inter-twilight period using the earliest and latest instances of civil twilight for each location across the entire reporting period (approximately 4:54 pm to 9:30 pm). shown in Figure 7, stops that occurred between the earliest end of civil twilight and the latest end of civil twilight would be included in the analyses. The blue line represents the end of civil twilight for a given day. Stops that occurred with sunlight fall under the blue line, while those without sunlight occurred above the blue line. The large dip in the trajectory of the blue line on November 4th is when the time switched from Daylight Savings Time back to Standard Time

[chekos]

related to #31