Bring a large pot three-fourths full of lightly salted water to a boil over medium-high heat.
Finely chop the basil, parsley, tarragon and sage and place in a bowl. Sprinkle in the lemon juice and stir to coat the herbs with the juice.
Combine the garlic, Parmesan and pine nuts in a blender or food processor. With the machine running, add the olive oil in a slow, steady stream and process until the mixture is creamy and blended. Pour the oil mixture into the bowl with the herbs and stir to combine.
Add the spaghetti to the boiling water and cook, stirring occasionally to prevent sticking, until the pasta is al dente, 10 to 12 minutes or according to package directions. Drain the pasta and transfer to a serving bowl. Toss with the pesto, add a pinch or two of salt and a few grinds of pepper. Top with the celery leaves and serve right away