They are both about being alone, but the difference is a state of mind.
According to merriam-webster.com:
Solitude (noun) is “a state or situation in which you are alone usually because you want to be.”
Lonely (adjective) as being “sad from being apart from other people.”
Language… has created the word “loneliness” to express the pain of being alone. And it has created the word “solitude” to express the glory of being alone.
– Paul Tillich
Here are what some people, including a genius or two, have to say about the glory of being alone.
“Without great solitude no serious work is possible.”
– Pablo Picasso
“The monotony and solitude of a quiet life stimulates the creative mind.”
– Albert Einstein
“The best thinking has been done in solitude.”
– Thomas Edison
In these times of rapid social changes, we can easily get detached and lonely, despite gadgets (and sometimes because of them) that are supposed to help us connect with others. As Henry David Thoreau said, “It is not enough to be busy. So are the ants. The question is: What are we busy about?”
In the song “Message in a Bottle,” Sting wrote of “a hundred billion bottles” with messages washed ashore and concluded “Seems I’m not alone at being alone, hundred billion castaways, looking for a home.”
“Not until we are lost do we begin to understand ourselves.”
– Henry David Thoreau
Perhaps we should pay more attention to Solitude. In the silence, hear our inner voice, find our way when we feel we are lost, see and appreciate the poetry in the universe.
Solitude. What separates it from Loneliness is a State of Mind.