The poem's personified earth is a kindly presence: a wise old man whose "good gigantic smile" suggests a grand embrace of life.
The earth, in this speaker's vision, is brown and lean, old and worn. The speaker's particular attention to the earth's "bones" stretched out to warm in the sun and to his "knees and feet" resting in the water paints a picture of a lanky old sailor, a guy who's seen enough of life to really appreciate the peace of a sunlit morning. At just the same time, these images evoke the knobbly, weather-beaten rocks of a cliff.
Where this personified earth sits, all is well. His "good gigantic smile" suggests there's something simply right about this day; the very waves on the beach ripple with "mirth," giddy laughter. The plainness of the word "good" and the vastness of the word "gigantic" suggest that the speaker perceives a fundamental, simple, and grand contentment in the earth's way of being.
She takes part in that contentment, too. She may be suffering, but in the sage, kindly company of the earth, she's not alone. Like an affectionate grandfather, "old earth smiles and knows," sympathizing: "life's trial," his wise contentment suggests, can't be dodged, but must be embraced.