Final answer:
Vehicle lights must be turned on by law from 30 minutes after sunset until 30 minutes before sunrise to ensure sufficient visibility. The use of headlights is a legal requirement not directly tied to the astronomical definition of twilight.
Step-by-step explanation:
By law, your vehicle's lights must be turned on from 30 minutes after sunset until 30 minutes before sunrise. This is to ensure that there is sufficient visibility for both the driver and for others on the road. Twilight, which provides some illumination, is defined by astronomers as the time when the Sun is 18° below the horizon both in the morning and evening.
However, the legal requirements for using vehicle headlights are not based on astronomical twilight but are clearly defined by traffic laws, typically the 30 minutes after sunset until 30 minutes before sunrise timeframe.
Regarding the movement of stars like Sirius, due to sidereal time being about 4 minutes shorter than solar time each day, a star like Sirius will rise approximately 2 hours earlier over the span of a month. Therefore, if Sirius rises at 7:00 p.m. today, it will rise approximately 6 hours earlier, around 1:00 p.m., which would be during daylight, in three months' time and would not be visible at its previous nighttime hours.
Therefore the correct Option is b. 30 minutes after sunset until 30 minutes before sunrise.