Get out the microscope, because we’re going through this poem line-by-line.
The glorious lamp of heaven, the sun,
The higher he's a-getting,
The sooner will his race be run,
And nearer he's to setting.
- The speaker continues with another example of the passage of time.
- The higher the sun gets (the further west it moves), the "sooner" will its journey be over, because it's "nearer" to "setting."
- The speaker calls the sun a "glorious lamp" because it gives off light.
- "A-getting" is just an older or poetic way of saying "getting."
- The sun isn't actually running a race; "race" can mean "journey, voyage, path."
- The progress of the sun through the sky, which is how we measure a day, recalls the first stanza's discussion of "today" and "tomorrow."